Annotation of gforth/gforth.ds, revision 1.18
1.1 anton 1: \input texinfo @c -*-texinfo-*-
2: @comment The source is gforth.ds, from which gforth.texi is generated
3: @comment %**start of header (This is for running Texinfo on a region.)
1.4 anton 4: @setfilename gforth.info
1.17 anton 5: @settitle Gforth Manual
1.4 anton 6: @comment @setchapternewpage odd
1.1 anton 7: @comment %**end of header (This is for running Texinfo on a region.)
8:
9: @ifinfo
1.17 anton 10: This file documents Gforth 0.1
1.1 anton 11:
1.17 anton 12: Copyright @copyright{} 1994 Gforth Development Group
1.1 anton 13:
14: Permission is granted to make and distribute verbatim copies of
15: this manual provided the copyright notice and this permission notice
16: are preserved on all copies.
17:
1.4 anton 18: @ignore
1.1 anton 19: Permission is granted to process this file through TeX and print the
20: results, provided the printed document carries a copying permission
21: notice identical to this one except for the removal of this paragraph
22: (this paragraph not being relevant to the printed manual).
23:
1.4 anton 24: @end ignore
1.1 anton 25: Permission is granted to copy and distribute modified versions of this
26: manual under the conditions for verbatim copying, provided also that the
27: sections entitled "Distribution" and "General Public License" are
28: included exactly as in the original, and provided that the entire
29: resulting derived work is distributed under the terms of a permission
30: notice identical to this one.
31:
32: Permission is granted to copy and distribute translations of this manual
33: into another language, under the above conditions for modified versions,
34: except that the sections entitled "Distribution" and "General Public
35: License" may be included in a translation approved by the author instead
36: of in the original English.
37: @end ifinfo
38:
39: @titlepage
40: @sp 10
1.17 anton 41: @center @titlefont{Gforth Manual}
1.1 anton 42: @sp 2
1.17 anton 43: @center for version 0.1
1.1 anton 44: @sp 2
45: @center Anton Ertl
1.17 anton 46: @sp 3
47: @center This manual is under construction
1.1 anton 48:
49: @comment The following two commands start the copyright page.
50: @page
51: @vskip 0pt plus 1filll
1.17 anton 52: Copyright @copyright{} 1994 Gforth Development Group
1.1 anton 53:
54: @comment !! Published by ... or You can get a copy of this manual ...
55:
56: Permission is granted to make and distribute verbatim copies of
57: this manual provided the copyright notice and this permission notice
58: are preserved on all copies.
59:
60: Permission is granted to copy and distribute modified versions of this
61: manual under the conditions for verbatim copying, provided also that the
62: sections entitled "Distribution" and "General Public License" are
63: included exactly as in the original, and provided that the entire
64: resulting derived work is distributed under the terms of a permission
65: notice identical to this one.
66:
67: Permission is granted to copy and distribute translations of this manual
68: into another language, under the above conditions for modified versions,
69: except that the sections entitled "Distribution" and "General Public
70: License" may be included in a translation approved by the author instead
71: of in the original English.
72: @end titlepage
73:
74:
75: @node Top, License, (dir), (dir)
76: @ifinfo
1.17 anton 77: Gforth is a free implementation of ANS Forth available on many
1.1 anton 78: personal machines. This manual corresponds to version 0.0.
79: @end ifinfo
80:
81: @menu
1.4 anton 82: * License::
1.17 anton 83: * Goals:: About the Gforth Project
1.4 anton 84: * Other Books:: Things you might want to read
1.17 anton 85: * Invocation:: Starting Gforth
86: * Words:: Forth words available in Gforth
1.4 anton 87: * ANS conformance:: Implementation-defined options etc.
1.17 anton 88: * Model:: The abstract machine of Gforth
89: * Emacs and Gforth:: The Gforth Mode
1.4 anton 90: * Internals:: Implementation details
91: * Bugs:: How to report them
1.17 anton 92: * Pedigree:: Ancestors of Gforth
1.4 anton 93: * Word Index:: An item for each Forth word
94: * Node Index:: An item for each node
1.1 anton 95: @end menu
96:
97: @node License, Goals, Top, Top
98: @unnumbered License
99: !! Insert GPL here
100:
101: @iftex
102: @unnumbered Preface
1.17 anton 103: This manual documents Gforth. The reader is expected to know
1.1 anton 104: Forth. This manual is primarily a reference manual. @xref{Other Books}
105: for introductory material.
106: @end iftex
107:
108: @node Goals, Other Books, License, Top
109: @comment node-name, next, previous, up
1.17 anton 110: @chapter Goals of Gforth
1.1 anton 111: @cindex Goals
1.17 anton 112: The goal of the Gforth Project is to develop a standard model for
1.1 anton 113: ANSI Forth. This can be split into several subgoals:
114:
115: @itemize @bullet
116: @item
1.17 anton 117: Gforth should conform to the ANSI Forth standard.
1.1 anton 118: @item
119: It should be a model, i.e. it should define all the
120: implementation-dependent things.
121: @item
122: It should become standard, i.e. widely accepted and used. This goal
123: is the most difficult one.
124: @end itemize
125:
1.17 anton 126: To achieve these goals Gforth should be
1.1 anton 127: @itemize @bullet
128: @item
129: Similar to previous models (fig-Forth, F83)
130: @item
131: Powerful. It should provide for all the things that are considered
132: necessary today and even some that are not yet considered necessary.
133: @item
134: Efficient. It should not get the reputation of being exceptionally
135: slow.
136: @item
137: Free.
138: @item
139: Available on many machines/easy to port.
140: @end itemize
141:
1.17 anton 142: Have we achieved these goals? Gforth conforms to the ANS Forth
143: standard. It may be considered a model, but we have not yet documented
1.1 anton 144: which parts of the model are stable and which parts we are likely to
1.17 anton 145: change. It certainly has not yet become a de facto standard. It has some
146: similarities and some differences to previous models. It has some
147: powerful features, but not yet everything that we envisioned. We
148: certainly have achieved our execution speed goals (@pxref{Performance}).
149: It is free and available on many machines.
1.1 anton 150:
151: @node Other Books, Invocation, Goals, Top
152: @chapter Other books on ANS Forth
153:
154: As the standard is relatively new, there are not many books out yet. It
1.17 anton 155: is not recommended to learn Forth by using Gforth and a book that is
1.1 anton 156: not written for ANS Forth, as you will not know your mistakes from the
157: deviations of the book.
158:
159: There is, of course, the standard, the definite reference if you want to
160: write ANS Forth programs. It will be available in printed form from
161: Global Engineering Documents !! somtime in spring or summer 1994. If you
162: are lucky, you can still get dpANS6 (the draft that was approved as
163: standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14.
164:
165: @cite{Forth: The new model} by Jack Woehr (!! Publisher) is an
166: introductory book based on a draft version of the standard. It does not
167: cover the whole standard. It also contains interesting background
168: information (Jack Woehr was in the ANS Forth Technical Committe). It is
169: not appropriate for complete newbies, but programmers experienced in
170: other languages should find it ok.
171:
172: @node Invocation, Words, Other Books, Top
173: @chapter Invocation
174:
175: You will usually just say @code{gforth}. In many other cases the default
1.17 anton 176: Gforth image will be invoked like this:
1.1 anton 177:
178: @example
179: gforth [files] [-e forth-code]
180: @end example
181:
182: executing the contents of the files and the Forth code in the order they
183: are given.
184:
185: In general, the command line looks like this:
186:
187: @example
188: gforth [initialization options] [image-specific options]
189: @end example
190:
191: The initialization options must come before the rest of the command
192: line. They are:
193:
194: @table @code
195: @item --image-file @var{file}
196: Loads the Forth image @var{file} instead of the default
197: @file{gforth.fi}.
198:
199: @item --path @var{path}
200: Uses @var{path} for searching the image file and Forth source code
201: files instead of the default in the environment variable
202: @code{GFORTHPATH} or the path specified at installation time (typically
203: @file{/usr/local/lib/gforth:.}). A path is given as a @code{:}-separated
204: list.
205:
206: @item --dictionary-size @var{size}
207: @item -m @var{size}
208: Allocate @var{size} space for the Forth dictionary space instead of
209: using the default specified in the image (typically 256K). The
210: @var{size} specification consists of an integer and a unit (e.g.,
211: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
212: size, in this case Cells), @code{k} (kilobytes), and @code{M}
213: (Megabytes). If no unit is specified, @code{e} is used.
214:
215: @item --data-stack-size @var{size}
216: @item -d @var{size}
217: Allocate @var{size} space for the data stack instead of using the
218: default specified in the image (typically 16K).
219:
220: @item --return-stack-size @var{size}
221: @item -r @var{size}
222: Allocate @var{size} space for the return stack instead of using the
223: default specified in the image (typically 16K).
224:
225: @item --fp-stack-size @var{size}
226: @item -f @var{size}
227: Allocate @var{size} space for the floating point stack instead of
228: using the default specified in the image (typically 16K). In this case
229: the unit specifier @code{e} refers to floating point numbers.
230:
231: @item --locals-stack-size @var{size}
232: @item -l @var{size}
233: Allocate @var{size} space for the locals stack instead of using the
234: default specified in the image (typically 16K).
235:
236: @end table
237:
238: As explained above, the image-specific command-line arguments for the
239: default image @file{gforth.fi} consist of a sequence of filenames and
240: @code{-e @var{forth-code}} options that are interpreted in the seqence
241: in which they are given. The @code{-e @var{forth-code}} or
242: @code{--evaluate @var{forth-code}} option evaluates the forth
243: code. This option takes only one argument; if you want to evaluate more
244: Forth words, you have to quote them or use several @code{-e}s. To exit
245: after processing the command line (instead of entering interactive mode)
246: append @code{-e bye} to the command line.
247:
248: Not yet implemented:
249: On startup the system first executes the system initialization file
250: (unless the option @code{--no-init-file} is given; note that the system
251: resulting from using this option may not be ANS Forth conformant). Then
252: the user initialization file @file{.gforth.fs} is executed, unless the
253: option @code{--no-rc} is given; this file is first searched in @file{.},
254: then in @file{~}, then in the normal path (see above).
255:
1.4 anton 256: @node Words, ANS conformance, Invocation, Top
1.1 anton 257: @chapter Forth Words
258:
259: @menu
1.4 anton 260: * Notation::
261: * Arithmetic::
262: * Stack Manipulation::
263: * Memory access::
264: * Control Structures::
265: * Locals::
266: * Defining Words::
267: * Wordlists::
268: * Files::
269: * Blocks::
270: * Other I/O::
271: * Programming Tools::
1.18 ! anton 272: * Assembler and Code words::
1.4 anton 273: * Threading Words::
1.1 anton 274: @end menu
275:
276: @node Notation, Arithmetic, Words, Words
277: @section Notation
278:
279: The Forth words are described in this section in the glossary notation
280: that has become a de-facto standard for Forth texts, i.e.
281:
1.4 anton 282: @format
1.1 anton 283: @var{word} @var{Stack effect} @var{wordset} @var{pronunciation}
1.4 anton 284: @end format
1.1 anton 285: @var{Description}
286:
287: @table @var
288: @item word
1.17 anton 289: The name of the word. BTW, Gforth is case insensitive, so you can
1.14 anton 290: type the words in in lower case (However, @pxref{core-idef}).
1.1 anton 291:
292: @item Stack effect
293: The stack effect is written in the notation @code{@var{before} --
294: @var{after}}, where @var{before} and @var{after} describe the top of
295: stack entries before and after the execution of the word. The rest of
296: the stack is not touched by the word. The top of stack is rightmost,
1.17 anton 297: i.e., a stack sequence is written as it is typed in. Note that Gforth
1.1 anton 298: uses a separate floating point stack, but a unified stack
299: notation. Also, return stack effects are not shown in @var{stack
300: effect}, but in @var{Description}. The name of a stack item describes
301: the type and/or the function of the item. See below for a discussion of
302: the types.
303:
304: @item pronunciation
305: How the word is pronounced
306:
307: @item wordset
308: The ANS Forth standard is divided into several wordsets. A standard
309: system need not support all of them. So, the fewer wordsets your program
310: uses the more portable it will be in theory. However, we suspect that
311: most ANS Forth systems on personal machines will feature all
312: wordsets. Words that are not defined in the ANS standard have
313: @code{gforth} as wordset.
314:
315: @item Description
316: A description of the behaviour of the word.
317: @end table
318:
1.4 anton 319: The type of a stack item is specified by the character(s) the name
320: starts with:
1.1 anton 321:
322: @table @code
323: @item f
324: Bool, i.e. @code{false} or @code{true}.
325: @item c
326: Char
327: @item w
328: Cell, can contain an integer or an address
329: @item n
330: signed integer
331: @item u
332: unsigned integer
333: @item d
334: double sized signed integer
335: @item ud
336: double sized unsigned integer
337: @item r
338: Float
339: @item a_
340: Cell-aligned address
341: @item c_
342: Char-aligned address (note that a Char is two bytes in Windows NT)
343: @item f_
344: Float-aligned address
345: @item df_
346: Address aligned for IEEE double precision float
347: @item sf_
348: Address aligned for IEEE single precision float
349: @item xt
350: Execution token, same size as Cell
351: @item wid
352: Wordlist ID, same size as Cell
353: @item f83name
354: Pointer to a name structure
355: @end table
356:
1.4 anton 357: @node Arithmetic, Stack Manipulation, Notation, Words
1.1 anton 358: @section Arithmetic
359: Forth arithmetic is not checked, i.e., you will not hear about integer
360: overflow on addition or multiplication, you may hear about division by
361: zero if you are lucky. The operator is written after the operands, but
362: the operands are still in the original order. I.e., the infix @code{2-1}
363: corresponds to @code{2 1 -}. Forth offers a variety of division
364: operators. If you perform division with potentially negative operands,
365: you do not want to use @code{/} or @code{/mod} with its undefined
366: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
1.4 anton 367: former, @pxref{Mixed precision}).
368:
369: @menu
370: * Single precision::
371: * Bitwise operations::
372: * Mixed precision:: operations with single and double-cell integers
373: * Double precision:: Double-cell integer arithmetic
374: * Floating Point::
375: @end menu
1.1 anton 376:
1.4 anton 377: @node Single precision, Bitwise operations, Arithmetic, Arithmetic
1.1 anton 378: @subsection Single precision
379: doc-+
380: doc--
381: doc-*
382: doc-/
383: doc-mod
384: doc-/mod
385: doc-negate
386: doc-abs
387: doc-min
388: doc-max
389:
1.4 anton 390: @node Bitwise operations, Mixed precision, Single precision, Arithmetic
1.1 anton 391: @subsection Bitwise operations
392: doc-and
393: doc-or
394: doc-xor
395: doc-invert
396: doc-2*
397: doc-2/
398:
1.4 anton 399: @node Mixed precision, Double precision, Bitwise operations, Arithmetic
1.1 anton 400: @subsection Mixed precision
401: doc-m+
402: doc-*/
403: doc-*/mod
404: doc-m*
405: doc-um*
406: doc-m*/
407: doc-um/mod
408: doc-fm/mod
409: doc-sm/rem
410:
1.4 anton 411: @node Double precision, Floating Point, Mixed precision, Arithmetic
1.1 anton 412: @subsection Double precision
1.16 anton 413:
414: The outer (aka text) interpreter converts numbers containing a dot into
415: a double precision number. Note that only numbers with the dot as last
416: character are standard-conforming.
417:
1.1 anton 418: doc-d+
419: doc-d-
420: doc-dnegate
421: doc-dabs
422: doc-dmin
423: doc-dmax
424:
1.4 anton 425: @node Floating Point, , Double precision, Arithmetic
426: @subsection Floating Point
1.16 anton 427:
428: The format of floating point numbers recognized by the outer (aka text)
429: interpreter is: a signed decimal number, possibly containing a decimal
430: point (@code{.}), followed by @code{E} or @code{e}, optionally followed
431: by a signed integer (the exponent). E.g., @code{1e} ist the same as
432: @code{+1.0e+1}. Note that a number without @code{e}
433: is not interpreted as floating-point number, but as double (if the
434: number contains a @code{.}) or single precision integer. Also,
435: conversions between string and floating point numbers always use base
436: 10, irrespective of the value of @code{BASE}. If @code{BASE} contains a
437: value greater then 14, the @code{E} may be interpreted as digit and the
438: number will be interpreted as integer, unless it has a signed exponent
439: (both @code{+} and @code{-} are allowed as signs).
1.4 anton 440:
441: Angles in floating point operations are given in radians (a full circle
1.17 anton 442: has 2 pi radians). Note, that Gforth has a separate floating point
1.4 anton 443: stack, but we use the unified notation.
444:
445: Floating point numbers have a number of unpleasant surprises for the
446: unwary (e.g., floating point addition is not associative) and even a few
447: for the wary. You should not use them unless you know what you are doing
448: or you don't care that the results you get are totally bogus. If you
449: want to learn about the problems of floating point numbers (and how to
1.11 anton 450: avoid them), you might start with @cite{David Goldberg, What Every
1.6 anton 451: Computer Scientist Should Know About Floating-Point Arithmetic, ACM
452: Computing Surveys 23(1):5@minus{}48, March 1991}.
1.4 anton 453:
454: doc-f+
455: doc-f-
456: doc-f*
457: doc-f/
458: doc-fnegate
459: doc-fabs
460: doc-fmax
461: doc-fmin
462: doc-floor
463: doc-fround
464: doc-f**
465: doc-fsqrt
466: doc-fexp
467: doc-fexpm1
468: doc-fln
469: doc-flnp1
470: doc-flog
1.6 anton 471: doc-falog
1.4 anton 472: doc-fsin
473: doc-fcos
474: doc-fsincos
475: doc-ftan
476: doc-fasin
477: doc-facos
478: doc-fatan
479: doc-fatan2
480: doc-fsinh
481: doc-fcosh
482: doc-ftanh
483: doc-fasinh
484: doc-facosh
485: doc-fatanh
486:
487: @node Stack Manipulation, Memory access, Arithmetic, Words
1.1 anton 488: @section Stack Manipulation
489:
1.17 anton 490: Gforth has a data stack (aka parameter stack) for characters, cells,
1.1 anton 491: addresses, and double cells, a floating point stack for floating point
492: numbers, a return stack for storing the return addresses of colon
493: definitions and other data, and a locals stack for storing local
494: variables. Note that while every sane Forth has a separate floating
495: point stack, this is not strictly required; an ANS Forth system could
496: theoretically keep floating point numbers on the data stack. As an
497: additional difficulty, you don't know how many cells a floating point
498: number takes. It is reportedly possible to write words in a way that
499: they work also for a unified stack model, but we do not recommend trying
1.4 anton 500: it. Instead, just say that your program has an environmental dependency
501: on a separate FP stack.
502:
503: Also, a Forth system is allowed to keep the local variables on the
1.1 anton 504: return stack. This is reasonable, as local variables usually eliminate
505: the need to use the return stack explicitly. So, if you want to produce
506: a standard complying program and if you are using local variables in a
507: word, forget about return stack manipulations in that word (see the
508: standard document for the exact rules).
509:
1.4 anton 510: @menu
511: * Data stack::
512: * Floating point stack::
513: * Return stack::
514: * Locals stack::
515: * Stack pointer manipulation::
516: @end menu
517:
518: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
1.1 anton 519: @subsection Data stack
520: doc-drop
521: doc-nip
522: doc-dup
523: doc-over
524: doc-tuck
525: doc-swap
526: doc-rot
527: doc--rot
528: doc-?dup
529: doc-pick
530: doc-roll
531: doc-2drop
532: doc-2nip
533: doc-2dup
534: doc-2over
535: doc-2tuck
536: doc-2swap
537: doc-2rot
538:
1.4 anton 539: @node Floating point stack, Return stack, Data stack, Stack Manipulation
1.1 anton 540: @subsection Floating point stack
541: doc-fdrop
542: doc-fnip
543: doc-fdup
544: doc-fover
545: doc-ftuck
546: doc-fswap
547: doc-frot
548:
1.4 anton 549: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
1.1 anton 550: @subsection Return stack
551: doc->r
552: doc-r>
553: doc-r@
554: doc-rdrop
555: doc-2>r
556: doc-2r>
557: doc-2r@
558: doc-2rdrop
559:
1.4 anton 560: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
1.1 anton 561: @subsection Locals stack
562:
1.4 anton 563: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
1.1 anton 564: @subsection Stack pointer manipulation
565: doc-sp@
566: doc-sp!
567: doc-fp@
568: doc-fp!
569: doc-rp@
570: doc-rp!
571: doc-lp@
572: doc-lp!
573:
1.4 anton 574: @node Memory access, Control Structures, Stack Manipulation, Words
1.1 anton 575: @section Memory access
576:
1.4 anton 577: @menu
578: * Stack-Memory transfers::
579: * Address arithmetic::
580: * Memory block access::
581: @end menu
582:
583: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
1.1 anton 584: @subsection Stack-Memory transfers
585:
586: doc-@
587: doc-!
588: doc-+!
589: doc-c@
590: doc-c!
591: doc-2@
592: doc-2!
593: doc-f@
594: doc-f!
595: doc-sf@
596: doc-sf!
597: doc-df@
598: doc-df!
599:
1.4 anton 600: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
1.1 anton 601: @subsection Address arithmetic
602:
603: ANS Forth does not specify the sizes of the data types. Instead, it
604: offers a number of words for computing sizes and doing address
605: arithmetic. Basically, address arithmetic is performed in terms of
606: address units (aus); on most systems the address unit is one byte. Note
607: that a character may have more than one au, so @code{chars} is no noop
608: (on systems where it is a noop, it compiles to nothing).
609:
610: ANS Forth also defines words for aligning addresses for specific
611: addresses. Many computers require that accesses to specific data types
612: must only occur at specific addresses; e.g., that cells may only be
613: accessed at addresses divisible by 4. Even if a machine allows unaligned
614: accesses, it can usually perform aligned accesses faster.
615:
1.17 anton 616: For the performance-conscious: alignment operations are usually only
1.1 anton 617: necessary during the definition of a data structure, not during the
618: (more frequent) accesses to it.
619:
620: ANS Forth defines no words for character-aligning addresses. This is not
621: an oversight, but reflects the fact that addresses that are not
622: char-aligned have no use in the standard and therefore will not be
623: created.
624:
625: The standard guarantees that addresses returned by @code{CREATE}d words
1.17 anton 626: are cell-aligned; in addition, Gforth guarantees that these addresses
1.1 anton 627: are aligned for all purposes.
628:
1.9 anton 629: Note that the standard defines a word @code{char}, which has nothing to
630: do with address arithmetic.
631:
1.1 anton 632: doc-chars
633: doc-char+
634: doc-cells
635: doc-cell+
636: doc-align
637: doc-aligned
638: doc-floats
639: doc-float+
640: doc-falign
641: doc-faligned
642: doc-sfloats
643: doc-sfloat+
644: doc-sfalign
645: doc-sfaligned
646: doc-dfloats
647: doc-dfloat+
648: doc-dfalign
649: doc-dfaligned
1.10 anton 650: doc-maxalign
651: doc-maxaligned
652: doc-cfalign
653: doc-cfaligned
1.1 anton 654: doc-address-unit-bits
655:
1.4 anton 656: @node Memory block access, , Address arithmetic, Memory access
1.1 anton 657: @subsection Memory block access
658:
659: doc-move
660: doc-erase
661:
662: While the previous words work on address units, the rest works on
663: characters.
664:
665: doc-cmove
666: doc-cmove>
667: doc-fill
668: doc-blank
669:
1.4 anton 670: @node Control Structures, Locals, Memory access, Words
1.1 anton 671: @section Control Structures
672:
673: Control structures in Forth cannot be used in interpret state, only in
674: compile state, i.e., in a colon definition. We do not like this
675: limitation, but have not seen a satisfying way around it yet, although
676: many schemes have been proposed.
677:
1.4 anton 678: @menu
679: * Selection::
680: * Simple Loops::
681: * Counted Loops::
682: * Arbitrary control structures::
683: * Calls and returns::
684: * Exception Handling::
685: @end menu
686:
687: @node Selection, Simple Loops, Control Structures, Control Structures
1.1 anton 688: @subsection Selection
689:
690: @example
691: @var{flag}
692: IF
693: @var{code}
694: ENDIF
695: @end example
696: or
697: @example
698: @var{flag}
699: IF
700: @var{code1}
701: ELSE
702: @var{code2}
703: ENDIF
704: @end example
705:
1.4 anton 706: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
1.1 anton 707: standard, and @code{ENDIF} is not, although it is quite popular. We
708: recommend using @code{ENDIF}, because it is less confusing for people
709: who also know other languages (and is not prone to reinforcing negative
710: prejudices against Forth in these people). Adding @code{ENDIF} to a
711: system that only supplies @code{THEN} is simple:
712: @example
713: : endif POSTPONE then ; immediate
714: @end example
715:
716: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
717: (adv.)} has the following meanings:
718: @quotation
719: ... 2b: following next after in order ... 3d: as a necessary consequence
720: (if you were there, then you saw them).
721: @end quotation
722: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
723: and many other programming languages has the meaning 3d.]
724:
725: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
726: can avoid using @code{?dup}.
727:
728: @example
729: @var{n}
730: CASE
731: @var{n1} OF @var{code1} ENDOF
732: @var{n2} OF @var{code2} ENDOF
1.4 anton 733: @dots{}
1.1 anton 734: ENDCASE
735: @end example
736:
737: Executes the first @var{codei}, where the @var{ni} is equal to
738: @var{n}. A default case can be added by simply writing the code after
739: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
740: but must not consume it.
741:
1.4 anton 742: @node Simple Loops, Counted Loops, Selection, Control Structures
1.1 anton 743: @subsection Simple Loops
744:
745: @example
746: BEGIN
747: @var{code1}
748: @var{flag}
749: WHILE
750: @var{code2}
751: REPEAT
752: @end example
753:
754: @var{code1} is executed and @var{flag} is computed. If it is true,
755: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
756:
757: @example
758: BEGIN
759: @var{code}
760: @var{flag}
761: UNTIL
762: @end example
763:
764: @var{code} is executed. The loop is restarted if @code{flag} is false.
765:
766: @example
767: BEGIN
768: @var{code}
769: AGAIN
770: @end example
771:
772: This is an endless loop.
773:
1.4 anton 774: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
1.1 anton 775: @subsection Counted Loops
776:
777: The basic counted loop is:
778: @example
779: @var{limit} @var{start}
780: ?DO
781: @var{body}
782: LOOP
783: @end example
784:
785: This performs one iteration for every integer, starting from @var{start}
786: and up to, but excluding @var{limit}. The counter, aka index, can be
787: accessed with @code{i}. E.g., the loop
788: @example
789: 10 0 ?DO
790: i .
791: LOOP
792: @end example
793: prints
794: @example
795: 0 1 2 3 4 5 6 7 8 9
796: @end example
797: The index of the innermost loop can be accessed with @code{i}, the index
798: of the next loop with @code{j}, and the index of the third loop with
799: @code{k}.
800:
801: The loop control data are kept on the return stack, so there are some
802: restrictions on mixing return stack accesses and counted loop
803: words. E.g., if you put values on the return stack outside the loop, you
804: cannot read them inside the loop. If you put values on the return stack
805: within a loop, you have to remove them before the end of the loop and
806: before accessing the index of the loop.
807:
808: There are several variations on the counted loop:
809:
810: @code{LEAVE} leaves the innermost counted loop immediately.
811:
1.18 ! anton 812: If @var{start} is greater than @var{limit}, a @code{?DO} loop is entered
! 813: (and @code{LOOP} iterates until they become equal by wrap-around
! 814: arithmetic). This behaviour is usually not what you want. Therefore,
! 815: Gforth offers @code{+DO} and @code{U+DO} (as replacements for
! 816: @code{?DO}), which do not enter the loop if @var{start} is greater than
! 817: @var{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for
! 818: unsigned loop parameters. These words can be implemented easily on
! 819: standard systems, so using them does not make your programs hard to
! 820: port; e.g.:
! 821: @example
! 822: : +DO ( compile-time: -- do-sys; run-time: n1 n2 -- )
! 823: POSTPONE over POSTPONE min POSTPONE ?DO ; immediate
! 824: @end example
! 825:
1.1 anton 826: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
827: index by @var{n} instead of by 1. The loop is terminated when the border
828: between @var{limit-1} and @var{limit} is crossed. E.g.:
829:
1.18 ! anton 830: @code{4 0 +DO i . 2 +LOOP} prints @code{0 2}
1.1 anton 831:
1.18 ! anton 832: @code{4 1 +DO i . 2 +LOOP} prints @code{1 3}
1.1 anton 833:
834: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
835:
1.2 anton 836: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
1.1 anton 837:
1.2 anton 838: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
1.1 anton 839:
1.18 ! anton 840: Therefore we recommend avoiding @code{@var{n} +LOOP} with negative
! 841: @var{n}. One alternative is @code{@var{u} -LOOP}, which reduces the
! 842: index by @var{u} each iteration. The loop is terminated when the border
! 843: between @var{limit+1} and @var{limit} is crossed. Gforth also provides
! 844: @code{-DO} and @code{U-DO} for down-counting loops. E.g.:
1.1 anton 845:
1.18 ! anton 846: @code{-2 0 -DO i . 1 -LOOP} prints @code{0 -1}
1.1 anton 847:
1.18 ! anton 848: @code{-1 0 -DO i . 1 -LOOP} prints @code{0}
1.1 anton 849:
1.18 ! anton 850: @code{ 0 0 -DO i . 1 -LOOP} prints nothing
1.1 anton 851:
1.18 ! anton 852: Another alternative is @code{@var{n} S+LOOP}, where the negative
! 853: case behaves symmetrical to the positive case:
1.1 anton 854:
1.18 ! anton 855: @code{-2 0 -DO i . -1 S+LOOP} prints @code{0 -1}
! 856:
! 857: The loop is terminated when the border between @var{limit@minus{}sgn(n)}
! 858: and @var{limit} is crossed. Unfortunately, neither @code{-LOOP} nor
! 859: @code{S+LOOP} are part of the ANS Forth standard, and they are not easy
! 860: to implement using standard words. If you want to write standard
! 861: programs, just avoid counting down.
! 862:
! 863: @code{?DO} can also be replaced by @code{DO}. @code{DO} always enters
! 864: the loop, independent of the loop parameters. Do not use @code{DO}, even
! 865: if you know that the loop is entered in any case. Such knowledge tends
! 866: to become invalid during maintenance of a program, and then the
! 867: @code{DO} will make trouble.
1.1 anton 868:
869: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
870: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
871: return stack so @code{EXIT} can get to its return address.
872:
873: Another counted loop is
874: @example
875: @var{n}
876: FOR
877: @var{body}
878: NEXT
879: @end example
880: This is the preferred loop of native code compiler writers who are too
1.17 anton 881: lazy to optimize @code{?DO} loops properly. In Gforth, this loop
1.1 anton 882: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
883: and ending with 0. Other Forth systems may behave differently, even if
884: they support @code{FOR} loops.
885:
1.4 anton 886: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
1.2 anton 887: @subsection Arbitrary control structures
888:
889: ANS Forth permits and supports using control structures in a non-nested
890: way. Information about incomplete control structures is stored on the
891: control-flow stack. This stack may be implemented on the Forth data
1.17 anton 892: stack, and this is what we have done in Gforth.
1.2 anton 893:
894: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
895: entry represents a backward branch target. A few words are the basis for
896: building any control structure possible (except control structures that
897: need storage, like calls, coroutines, and backtracking).
898:
1.3 anton 899: doc-if
900: doc-ahead
901: doc-then
902: doc-begin
903: doc-until
904: doc-again
905: doc-cs-pick
906: doc-cs-roll
1.2 anton 907:
1.17 anton 908: On many systems control-flow stack items take one word, in Gforth they
1.2 anton 909: currently take three (this may change in the future). Therefore it is a
910: really good idea to manipulate the control flow stack with
911: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
912: words.
913:
914: Some standard control structure words are built from these words:
915:
1.3 anton 916: doc-else
917: doc-while
918: doc-repeat
1.2 anton 919:
920: Counted loop words constitute a separate group of words:
921:
1.3 anton 922: doc-?do
1.18 ! anton 923: doc-+do
! 924: doc-u+do
! 925: doc--do
! 926: doc-u-do
1.3 anton 927: doc-do
928: doc-for
929: doc-loop
930: doc-s+loop
931: doc-+loop
1.18 ! anton 932: doc--loop
1.3 anton 933: doc-next
934: doc-leave
935: doc-?leave
936: doc-unloop
1.10 anton 937: doc-done
1.2 anton 938:
939: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
940: @i{do-sys}. Our system allows it, but it's your job to ensure that for
941: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
1.3 anton 942: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
943: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
1.7 pazsan 944: resolved (by using one of the loop-ending words or @code{DONE}).
1.2 anton 945:
946: Another group of control structure words are
947:
1.3 anton 948: doc-case
949: doc-endcase
950: doc-of
951: doc-endof
1.2 anton 952:
953: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
954: @code{cs-roll}.
955:
1.3 anton 956: @subsubsection Programming Style
957:
958: In order to ensure readability we recommend that you do not create
959: arbitrary control structures directly, but define new control structure
960: words for the control structure you want and use these words in your
961: program.
962:
963: E.g., instead of writing
964:
965: @example
966: begin
967: ...
968: if [ 1 cs-roll ]
969: ...
970: again then
971: @end example
972:
973: we recommend defining control structure words, e.g.,
974:
975: @example
976: : while ( dest -- orig dest )
977: POSTPONE if
978: 1 cs-roll ; immediate
979:
980: : repeat ( orig dest -- )
981: POSTPONE again
982: POSTPONE then ; immediate
983: @end example
984:
985: and then using these to create the control structure:
986:
987: @example
988: begin
989: ...
990: while
991: ...
992: repeat
993: @end example
994:
995: That's much easier to read, isn't it? Of course, @code{BEGIN} and
996: @code{WHILE} are predefined, so in this example it would not be
997: necessary to define them.
998:
1.4 anton 999: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
1.3 anton 1000: @subsection Calls and returns
1001:
1002: A definition can be called simply be writing the name of the
1.17 anton 1003: definition. When the end of the definition is reached, it returns. An
1004: earlier return can be forced using
1.3 anton 1005:
1006: doc-exit
1007:
1008: Don't forget to clean up the return stack and @code{UNLOOP} any
1009: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
1010: primitive compiled by @code{EXIT} is
1011:
1012: doc-;s
1013:
1.4 anton 1014: @node Exception Handling, , Calls and returns, Control Structures
1.3 anton 1015: @subsection Exception Handling
1016:
1017: doc-catch
1018: doc-throw
1019:
1.4 anton 1020: @node Locals, Defining Words, Control Structures, Words
1.1 anton 1021: @section Locals
1022:
1.2 anton 1023: Local variables can make Forth programming more enjoyable and Forth
1024: programs easier to read. Unfortunately, the locals of ANS Forth are
1025: laden with restrictions. Therefore, we provide not only the ANS Forth
1026: locals wordset, but also our own, more powerful locals wordset (we
1027: implemented the ANS Forth locals wordset through our locals wordset).
1028:
1029: @menu
1.17 anton 1030: * Gforth locals::
1.4 anton 1031: * ANS Forth locals::
1.2 anton 1032: @end menu
1033:
1.17 anton 1034: @node Gforth locals, ANS Forth locals, Locals, Locals
1035: @subsection Gforth locals
1.2 anton 1036:
1037: Locals can be defined with
1038:
1039: @example
1040: @{ local1 local2 ... -- comment @}
1041: @end example
1042: or
1043: @example
1044: @{ local1 local2 ... @}
1045: @end example
1046:
1047: E.g.,
1048: @example
1049: : max @{ n1 n2 -- n3 @}
1050: n1 n2 > if
1051: n1
1052: else
1053: n2
1054: endif ;
1055: @end example
1056:
1057: The similarity of locals definitions with stack comments is intended. A
1058: locals definition often replaces the stack comment of a word. The order
1059: of the locals corresponds to the order in a stack comment and everything
1060: after the @code{--} is really a comment.
1061:
1062: This similarity has one disadvantage: It is too easy to confuse locals
1063: declarations with stack comments, causing bugs and making them hard to
1064: find. However, this problem can be avoided by appropriate coding
1065: conventions: Do not use both notations in the same program. If you do,
1066: they should be distinguished using additional means, e.g. by position.
1067:
1068: The name of the local may be preceded by a type specifier, e.g.,
1069: @code{F:} for a floating point value:
1070:
1071: @example
1072: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1073: \ complex multiplication
1074: Ar Br f* Ai Bi f* f-
1075: Ar Bi f* Ai Br f* f+ ;
1076: @end example
1077:
1.17 anton 1078: Gforth currently supports cells (@code{W:}, @code{W^}), doubles
1.2 anton 1079: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1080: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1081: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1082: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1083: produces its address (which becomes invalid when the variable's scope is
1084: left). E.g., the standard word @code{emit} can be defined in therms of
1085: @code{type} like this:
1086:
1087: @example
1088: : emit @{ C^ char* -- @}
1089: char* 1 type ;
1090: @end example
1091:
1092: A local without type specifier is a @code{W:} local. Both flavours of
1093: locals are initialized with values from the data or FP stack.
1094:
1095: Currently there is no way to define locals with user-defined data
1096: structures, but we are working on it.
1097:
1.17 anton 1098: Gforth allows defining locals everywhere in a colon definition. This
1.7 pazsan 1099: poses the following questions:
1.2 anton 1100:
1.4 anton 1101: @menu
1102: * Where are locals visible by name?::
1.14 anton 1103: * How long do locals live?::
1.4 anton 1104: * Programming Style::
1105: * Implementation::
1106: @end menu
1107:
1.17 anton 1108: @node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
1.2 anton 1109: @subsubsection Where are locals visible by name?
1110:
1111: Basically, the answer is that locals are visible where you would expect
1112: it in block-structured languages, and sometimes a little longer. If you
1113: want to restrict the scope of a local, enclose its definition in
1114: @code{SCOPE}...@code{ENDSCOPE}.
1115:
1116: doc-scope
1117: doc-endscope
1118:
1119: These words behave like control structure words, so you can use them
1120: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1121: arbitrary ways.
1122:
1123: If you want a more exact answer to the visibility question, here's the
1124: basic principle: A local is visible in all places that can only be
1125: reached through the definition of the local@footnote{In compiler
1126: construction terminology, all places dominated by the definition of the
1127: local.}. In other words, it is not visible in places that can be reached
1128: without going through the definition of the local. E.g., locals defined
1129: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1130: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1131: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1132:
1133: The reasoning behind this solution is: We want to have the locals
1134: visible as long as it is meaningful. The user can always make the
1135: visibility shorter by using explicit scoping. In a place that can
1136: only be reached through the definition of a local, the meaning of a
1137: local name is clear. In other places it is not: How is the local
1138: initialized at the control flow path that does not contain the
1139: definition? Which local is meant, if the same name is defined twice in
1140: two independent control flow paths?
1141:
1142: This should be enough detail for nearly all users, so you can skip the
1143: rest of this section. If you relly must know all the gory details and
1144: options, read on.
1145:
1146: In order to implement this rule, the compiler has to know which places
1147: are unreachable. It knows this automatically after @code{AHEAD},
1148: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1149: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1150: compiler that the control flow never reaches that place. If
1151: @code{UNREACHABLE} is not used where it could, the only consequence is
1152: that the visibility of some locals is more limited than the rule above
1153: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1154: lie to the compiler), buggy code will be produced.
1155:
1156: Another problem with this rule is that at @code{BEGIN}, the compiler
1.3 anton 1157: does not know which locals will be visible on the incoming
1158: back-edge. All problems discussed in the following are due to this
1159: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1160: loops as examples; the discussion also applies to @code{?DO} and other
1.2 anton 1161: loops). Perhaps the most insidious example is:
1162: @example
1163: AHEAD
1164: BEGIN
1165: x
1166: [ 1 CS-ROLL ] THEN
1.4 anton 1167: @{ x @}
1.2 anton 1168: ...
1169: UNTIL
1170: @end example
1171:
1172: This should be legal according to the visibility rule. The use of
1173: @code{x} can only be reached through the definition; but that appears
1174: textually below the use.
1175:
1176: From this example it is clear that the visibility rules cannot be fully
1177: implemented without major headaches. Our implementation treats common
1178: cases as advertised and the exceptions are treated in a safe way: The
1179: compiler makes a reasonable guess about the locals visible after a
1180: @code{BEGIN}; if it is too pessimistic, the
1181: user will get a spurious error about the local not being defined; if the
1182: compiler is too optimistic, it will notice this later and issue a
1183: warning. In the case above the compiler would complain about @code{x}
1184: being undefined at its use. You can see from the obscure examples in
1185: this section that it takes quite unusual control structures to get the
1186: compiler into trouble, and even then it will often do fine.
1187:
1188: If the @code{BEGIN} is reachable from above, the most optimistic guess
1189: is that all locals visible before the @code{BEGIN} will also be
1190: visible after the @code{BEGIN}. This guess is valid for all loops that
1191: are entered only through the @code{BEGIN}, in particular, for normal
1192: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1193: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1194: compiler. When the branch to the @code{BEGIN} is finally generated by
1195: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1196: warns the user if it was too optimisitic:
1197: @example
1198: IF
1.4 anton 1199: @{ x @}
1.2 anton 1200: BEGIN
1201: \ x ?
1202: [ 1 cs-roll ] THEN
1203: ...
1204: UNTIL
1205: @end example
1206:
1207: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1208: optimistically assumes that it lives until the @code{THEN}. It notices
1209: this difference when it compiles the @code{UNTIL} and issues a
1210: warning. The user can avoid the warning, and make sure that @code{x}
1211: is not used in the wrong area by using explicit scoping:
1212: @example
1213: IF
1214: SCOPE
1.4 anton 1215: @{ x @}
1.2 anton 1216: ENDSCOPE
1217: BEGIN
1218: [ 1 cs-roll ] THEN
1219: ...
1220: UNTIL
1221: @end example
1222:
1223: Since the guess is optimistic, there will be no spurious error messages
1224: about undefined locals.
1225:
1226: If the @code{BEGIN} is not reachable from above (e.g., after
1227: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1228: optimistic guess, as the locals visible after the @code{BEGIN} may be
1229: defined later. Therefore, the compiler assumes that no locals are
1.17 anton 1230: visible after the @code{BEGIN}. However, the user can use
1.2 anton 1231: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1.17 anton 1232: visible at the BEGIN as at the point where the top control-flow stack
1233: item was created.
1.2 anton 1234:
1235: doc-assume-live
1236:
1237: E.g.,
1238: @example
1.4 anton 1239: @{ x @}
1.2 anton 1240: AHEAD
1241: ASSUME-LIVE
1242: BEGIN
1243: x
1244: [ 1 CS-ROLL ] THEN
1245: ...
1246: UNTIL
1247: @end example
1248:
1249: Other cases where the locals are defined before the @code{BEGIN} can be
1250: handled by inserting an appropriate @code{CS-ROLL} before the
1251: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1252: behind the @code{ASSUME-LIVE}).
1253:
1254: Cases where locals are defined after the @code{BEGIN} (but should be
1255: visible immediately after the @code{BEGIN}) can only be handled by
1256: rearranging the loop. E.g., the ``most insidious'' example above can be
1257: arranged into:
1258: @example
1259: BEGIN
1.4 anton 1260: @{ x @}
1.2 anton 1261: ... 0=
1262: WHILE
1263: x
1264: REPEAT
1265: @end example
1266:
1.17 anton 1267: @node How long do locals live?, Programming Style, Where are locals visible by name?, Gforth locals
1.2 anton 1268: @subsubsection How long do locals live?
1269:
1270: The right answer for the lifetime question would be: A local lives at
1271: least as long as it can be accessed. For a value-flavoured local this
1272: means: until the end of its visibility. However, a variable-flavoured
1273: local could be accessed through its address far beyond its visibility
1274: scope. Ultimately, this would mean that such locals would have to be
1275: garbage collected. Since this entails un-Forth-like implementation
1276: complexities, I adopted the same cowardly solution as some other
1277: languages (e.g., C): The local lives only as long as it is visible;
1278: afterwards its address is invalid (and programs that access it
1279: afterwards are erroneous).
1280:
1.17 anton 1281: @node Programming Style, Implementation, How long do locals live?, Gforth locals
1.2 anton 1282: @subsubsection Programming Style
1283:
1284: The freedom to define locals anywhere has the potential to change
1285: programming styles dramatically. In particular, the need to use the
1286: return stack for intermediate storage vanishes. Moreover, all stack
1287: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1288: determined arguments) can be eliminated: If the stack items are in the
1289: wrong order, just write a locals definition for all of them; then
1290: write the items in the order you want.
1291:
1292: This seems a little far-fetched and eliminating stack manipulations is
1.4 anton 1293: unlikely to become a conscious programming objective. Still, the number
1294: of stack manipulations will be reduced dramatically if local variables
1.17 anton 1295: are used liberally (e.g., compare @code{max} in @ref{Gforth locals} with
1.4 anton 1296: a traditional implementation of @code{max}).
1.2 anton 1297:
1298: This shows one potential benefit of locals: making Forth programs more
1299: readable. Of course, this benefit will only be realized if the
1300: programmers continue to honour the principle of factoring instead of
1301: using the added latitude to make the words longer.
1302:
1303: Using @code{TO} can and should be avoided. Without @code{TO},
1304: every value-flavoured local has only a single assignment and many
1305: advantages of functional languages apply to Forth. I.e., programs are
1306: easier to analyse, to optimize and to read: It is clear from the
1307: definition what the local stands for, it does not turn into something
1308: different later.
1309:
1310: E.g., a definition using @code{TO} might look like this:
1311: @example
1312: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1313: u1 u2 min 0
1314: ?do
1315: addr1 c@ addr2 c@ - ?dup
1316: if
1317: unloop exit
1318: then
1319: addr1 char+ TO addr1
1320: addr2 char+ TO addr2
1321: loop
1322: u1 u2 - ;
1323: @end example
1324: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1325: every loop iteration. @code{strcmp} is a typical example of the
1326: readability problems of using @code{TO}. When you start reading
1327: @code{strcmp}, you think that @code{addr1} refers to the start of the
1328: string. Only near the end of the loop you realize that it is something
1329: else.
1330:
1331: This can be avoided by defining two locals at the start of the loop that
1332: are initialized with the right value for the current iteration.
1333: @example
1334: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1335: addr1 addr2
1336: u1 u2 min 0
1337: ?do @{ s1 s2 @}
1338: s1 c@ s2 c@ - ?dup
1339: if
1340: unloop exit
1341: then
1342: s1 char+ s2 char+
1343: loop
1344: 2drop
1345: u1 u2 - ;
1346: @end example
1347: Here it is clear from the start that @code{s1} has a different value
1348: in every loop iteration.
1349:
1.17 anton 1350: @node Implementation, , Programming Style, Gforth locals
1.2 anton 1351: @subsubsection Implementation
1352:
1.17 anton 1353: Gforth uses an extra locals stack. The most compelling reason for
1.2 anton 1354: this is that the return stack is not float-aligned; using an extra stack
1355: also eliminates the problems and restrictions of using the return stack
1356: as locals stack. Like the other stacks, the locals stack grows toward
1357: lower addresses. A few primitives allow an efficient implementation:
1358:
1359: doc-@local#
1360: doc-f@local#
1361: doc-laddr#
1362: doc-lp+!#
1363: doc-lp!
1364: doc->l
1365: doc-f>l
1366:
1367: In addition to these primitives, some specializations of these
1368: primitives for commonly occurring inline arguments are provided for
1369: efficiency reasons, e.g., @code{@@local0} as specialization of
1370: @code{@@local#} for the inline argument 0. The following compiling words
1371: compile the right specialized version, or the general version, as
1372: appropriate:
1373:
1.12 anton 1374: doc-compile-@local
1375: doc-compile-f@local
1.2 anton 1376: doc-compile-lp+!
1377:
1378: Combinations of conditional branches and @code{lp+!#} like
1379: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1380: is taken) are provided for efficiency and correctness in loops.
1381:
1382: A special area in the dictionary space is reserved for keeping the
1383: local variable names. @code{@{} switches the dictionary pointer to this
1384: area and @code{@}} switches it back and generates the locals
1385: initializing code. @code{W:} etc.@ are normal defining words. This
1386: special area is cleared at the start of every colon definition.
1387:
1.17 anton 1388: A special feature of Gforth's dictionary is used to implement the
1.2 anton 1389: definition of locals without type specifiers: every wordlist (aka
1390: vocabulary) has its own methods for searching
1.4 anton 1391: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1.2 anton 1392: with a special search method: When it is searched for a word, it
1393: actually creates that word using @code{W:}. @code{@{} changes the search
1394: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1395: and then the wordlist for defining locals without type specifiers.
1396:
1397: The lifetime rules support a stack discipline within a colon
1398: definition: The lifetime of a local is either nested with other locals
1399: lifetimes or it does not overlap them.
1400:
1401: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1402: pointer manipulation is generated. Between control structure words
1403: locals definitions can push locals onto the locals stack. @code{AGAIN}
1404: is the simplest of the other three control flow words. It has to
1405: restore the locals stack depth of the corresponding @code{BEGIN}
1406: before branching. The code looks like this:
1407: @format
1408: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1409: @code{branch} <begin>
1410: @end format
1411:
1412: @code{UNTIL} is a little more complicated: If it branches back, it
1413: must adjust the stack just like @code{AGAIN}. But if it falls through,
1414: the locals stack must not be changed. The compiler generates the
1415: following code:
1416: @format
1417: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1418: @end format
1419: The locals stack pointer is only adjusted if the branch is taken.
1420:
1421: @code{THEN} can produce somewhat inefficient code:
1422: @format
1423: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1424: <orig target>:
1425: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1426: @end format
1427: The second @code{lp+!#} adjusts the locals stack pointer from the
1.4 anton 1428: level at the @var{orig} point to the level after the @code{THEN}. The
1.2 anton 1429: first @code{lp+!#} adjusts the locals stack pointer from the current
1430: level to the level at the orig point, so the complete effect is an
1431: adjustment from the current level to the right level after the
1432: @code{THEN}.
1433:
1434: In a conventional Forth implementation a dest control-flow stack entry
1435: is just the target address and an orig entry is just the address to be
1436: patched. Our locals implementation adds a wordlist to every orig or dest
1437: item. It is the list of locals visible (or assumed visible) at the point
1438: described by the entry. Our implementation also adds a tag to identify
1439: the kind of entry, in particular to differentiate between live and dead
1440: (reachable and unreachable) orig entries.
1441:
1442: A few unusual operations have to be performed on locals wordlists:
1443:
1444: doc-common-list
1445: doc-sub-list?
1446: doc-list-size
1447:
1448: Several features of our locals wordlist implementation make these
1449: operations easy to implement: The locals wordlists are organised as
1450: linked lists; the tails of these lists are shared, if the lists
1451: contain some of the same locals; and the address of a name is greater
1452: than the address of the names behind it in the list.
1453:
1454: Another important implementation detail is the variable
1455: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1456: determine if they can be reached directly or only through the branch
1457: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1458: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1459: definition, by @code{BEGIN} and usually by @code{THEN}.
1460:
1461: Counted loops are similar to other loops in most respects, but
1462: @code{LEAVE} requires special attention: It performs basically the same
1463: service as @code{AHEAD}, but it does not create a control-flow stack
1464: entry. Therefore the information has to be stored elsewhere;
1465: traditionally, the information was stored in the target fields of the
1466: branches created by the @code{LEAVE}s, by organizing these fields into a
1467: linked list. Unfortunately, this clever trick does not provide enough
1468: space for storing our extended control flow information. Therefore, we
1469: introduce another stack, the leave stack. It contains the control-flow
1470: stack entries for all unresolved @code{LEAVE}s.
1471:
1472: Local names are kept until the end of the colon definition, even if
1473: they are no longer visible in any control-flow path. In a few cases
1474: this may lead to increased space needs for the locals name area, but
1475: usually less than reclaiming this space would cost in code size.
1476:
1477:
1.17 anton 1478: @node ANS Forth locals, , Gforth locals, Locals
1.2 anton 1479: @subsection ANS Forth locals
1480:
1481: The ANS Forth locals wordset does not define a syntax for locals, but
1482: words that make it possible to define various syntaxes. One of the
1.17 anton 1483: possible syntaxes is a subset of the syntax we used in the Gforth locals
1.2 anton 1484: wordset, i.e.:
1485:
1486: @example
1487: @{ local1 local2 ... -- comment @}
1488: @end example
1489: or
1490: @example
1491: @{ local1 local2 ... @}
1492: @end example
1493:
1494: The order of the locals corresponds to the order in a stack comment. The
1495: restrictions are:
1.1 anton 1496:
1.2 anton 1497: @itemize @bullet
1498: @item
1.17 anton 1499: Locals can only be cell-sized values (no type specifiers are allowed).
1.2 anton 1500: @item
1501: Locals can be defined only outside control structures.
1502: @item
1503: Locals can interfere with explicit usage of the return stack. For the
1504: exact (and long) rules, see the standard. If you don't use return stack
1.17 anton 1505: accessing words in a definition using locals, you will be all right. The
1.2 anton 1506: purpose of this rule is to make locals implementation on the return
1507: stack easier.
1508: @item
1509: The whole definition must be in one line.
1510: @end itemize
1511:
1512: Locals defined in this way behave like @code{VALUE}s
1.4 anton 1513: (@xref{Values}). I.e., they are initialized from the stack. Using their
1.2 anton 1514: name produces their value. Their value can be changed using @code{TO}.
1515:
1.17 anton 1516: Since this syntax is supported by Gforth directly, you need not do
1.2 anton 1517: anything to use it. If you want to port a program using this syntax to
1518: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1519: on the other system.
1520:
1521: Note that a syntax shown in the standard, section A.13 looks
1522: similar, but is quite different in having the order of locals
1523: reversed. Beware!
1524:
1525: The ANS Forth locals wordset itself consists of the following word
1526:
1527: doc-(local)
1528:
1529: The ANS Forth locals extension wordset defines a syntax, but it is so
1530: awful that we strongly recommend not to use it. We have implemented this
1.17 anton 1531: syntax to make porting to Gforth easy, but do not document it here. The
1.2 anton 1532: problem with this syntax is that the locals are defined in an order
1533: reversed with respect to the standard stack comment notation, making
1534: programs harder to read, and easier to misread and miswrite. The only
1535: merit of this syntax is that it is easy to implement using the ANS Forth
1536: locals wordset.
1.3 anton 1537:
1.4 anton 1538: @node Defining Words, Wordlists, Locals, Words
1539: @section Defining Words
1540:
1.14 anton 1541: @menu
1542: * Values::
1543: @end menu
1544:
1.4 anton 1545: @node Values, , Defining Words, Defining Words
1546: @subsection Values
1547:
1548: @node Wordlists, Files, Defining Words, Words
1549: @section Wordlists
1550:
1551: @node Files, Blocks, Wordlists, Words
1552: @section Files
1553:
1554: @node Blocks, Other I/O, Files, Words
1555: @section Blocks
1556:
1557: @node Other I/O, Programming Tools, Blocks, Words
1558: @section Other I/O
1559:
1.18 ! anton 1560: @node Programming Tools, Assembler and Code words, Other I/O, Words
1.4 anton 1561: @section Programming Tools
1562:
1.5 anton 1563: @menu
1564: * Debugging:: Simple and quick.
1565: * Assertions:: Making your programs self-checking.
1566: @end menu
1567:
1568: @node Debugging, Assertions, Programming Tools, Programming Tools
1.4 anton 1569: @subsection Debugging
1570:
1571: The simple debugging aids provided in @file{debugging.fs}
1572: are meant to support a different style of debugging than the
1573: tracing/stepping debuggers used in languages with long turn-around
1574: times.
1575:
1576: A much better (faster) way in fast-compilig languages is to add
1577: printing code at well-selected places, let the program run, look at
1578: the output, see where things went wrong, add more printing code, etc.,
1579: until the bug is found.
1580:
1581: The word @code{~~} is easy to insert. It just prints debugging
1582: information (by default the source location and the stack contents). It
1583: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1584: query-replace them with nothing). The deferred words
1585: @code{printdebugdata} and @code{printdebugline} control the output of
1586: @code{~~}. The default source location output format works well with
1587: Emacs' compilation mode, so you can step through the program at the
1.5 anton 1588: source level using @kbd{C-x `} (the advantage over a stepping debugger
1589: is that you can step in any direction and you know where the crash has
1590: happened or where the strange data has occurred).
1.4 anton 1591:
1592: Note that the default actions clobber the contents of the pictured
1593: numeric output string, so you should not use @code{~~}, e.g., between
1594: @code{<#} and @code{#>}.
1595:
1596: doc-~~
1597: doc-printdebugdata
1598: doc-printdebugline
1599:
1.5 anton 1600: @node Assertions, , Debugging, Programming Tools
1.4 anton 1601: @subsection Assertions
1602:
1.5 anton 1603: It is a good idea to make your programs self-checking, in particular, if
1604: you use an assumption (e.g., that a certain field of a data structure is
1.17 anton 1605: never zero) that may become wrong during maintenance. Gforth supports
1.5 anton 1606: assertions for this purpose. They are used like this:
1607:
1608: @example
1609: assert( @var{flag} )
1610: @end example
1611:
1612: The code between @code{assert(} and @code{)} should compute a flag, that
1613: should be true if everything is alright and false otherwise. It should
1614: not change anything else on the stack. The overall stack effect of the
1615: assertion is @code{( -- )}. E.g.
1616:
1617: @example
1618: assert( 1 1 + 2 = ) \ what we learn in school
1619: assert( dup 0<> ) \ assert that the top of stack is not zero
1620: assert( false ) \ this code should not be reached
1621: @end example
1622:
1623: The need for assertions is different at different times. During
1624: debugging, we want more checking, in production we sometimes care more
1625: for speed. Therefore, assertions can be turned off, i.e., the assertion
1626: becomes a comment. Depending on the importance of an assertion and the
1627: time it takes to check it, you may want to turn off some assertions and
1.17 anton 1628: keep others turned on. Gforth provides several levels of assertions for
1.5 anton 1629: this purpose:
1630:
1631: doc-assert0(
1632: doc-assert1(
1633: doc-assert2(
1634: doc-assert3(
1635: doc-assert(
1636: doc-)
1637:
1638: @code{Assert(} is the same as @code{assert1(}. The variable
1639: @code{assert-level} specifies the highest assertions that are turned
1640: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1641: @code{assert1(} assertions perform checking, while @code{assert2(} and
1642: @code{assert3(} assertions are treated as comments.
1643:
1644: Note that the @code{assert-level} is evaluated at compile-time, not at
1645: run-time. I.e., you cannot turn assertions on or off at run-time, you
1646: have to set the @code{assert-level} appropriately before compiling a
1647: piece of code. You can compile several pieces of code at several
1648: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1649: written code at level 3).
1650:
1651: doc-assert-level
1652:
1653: If an assertion fails, a message compatible with Emacs' compilation mode
1654: is produced and the execution is aborted (currently with @code{ABORT"}.
1655: If there is interest, we will introduce a special throw code. But if you
1656: intend to @code{catch} a specific condition, using @code{throw} is
1657: probably more appropriate than an assertion).
1658:
1.18 ! anton 1659: @node Assembler and Code words, Threading Words, Programming Tools, Words
! 1660: @section Assembler and Code words
! 1661:
! 1662: Gforth provides some words for defining primitives (words written in
! 1663: machine code), and for defining the the machine-code equivalent of
! 1664: @code{DOES>}-based defining words. However, the machine-independent
! 1665: nature of Gforth poses a few problems: First of all. Gforth runs on
! 1666: several architectures, so it can provide no standard assembler. What's
! 1667: worse is that the register allocation not only depends on the processor,
! 1668: but also on the gcc version and options used.
! 1669:
! 1670: The words Gforth offers encapsulate some system dependences (e.g., the
! 1671: header structure), so a system-independent assembler may be used in
! 1672: Gforth. If you do not have an assembler, you can compile machine code
! 1673: directly with @code{,} and @code{c,}.
! 1674:
! 1675: doc-assembler
! 1676: doc-code
! 1677: doc-end-code
! 1678: doc-;code
! 1679: doc-flush-icache
! 1680:
! 1681: If @code{flush-icache} does not work correctly, @code{code} words
! 1682: etc. will not work (reliably), either.
! 1683:
! 1684: These words are rarely used. Therefore they reside in @code{code.fs},
! 1685: which is usually not loaded (except @code{flush-icache}, which is always
! 1686: present). You can load it with @code{require code.fs}.
! 1687:
! 1688: Another option for implementing normal and defining words efficiently
! 1689: is: adding the wanted functionality to the source of Gforth. For normal
! 1690: words you just have to edit @file{primitives}, defining words (for fast
! 1691: defined words) probably require changes in @file{engine.c},
! 1692: @file{kernal.fs}, @file{prims2x.fs}, and possibly @file{cross.fs}.
! 1693:
! 1694:
! 1695: @node Threading Words, , Assembler and Code words, Words
1.4 anton 1696: @section Threading Words
1697:
1698: These words provide access to code addresses and other threading stuff
1.17 anton 1699: in Gforth (and, possibly, other interpretive Forths). It more or less
1.4 anton 1700: abstracts away the differences between direct and indirect threading
1701: (and, for direct threading, the machine dependences). However, at
1702: present this wordset is still inclomplete. It is also pretty low-level;
1703: some day it will hopefully be made unnecessary by an internals words set
1704: that abstracts implementation details away completely.
1705:
1706: doc->code-address
1707: doc->does-code
1708: doc-code-address!
1709: doc-does-code!
1710: doc-does-handler!
1711: doc-/does-handler
1712:
1.18 ! anton 1713: The code addresses produced by various defining words are produced by
! 1714: the following words:
1.14 anton 1715:
1.18 ! anton 1716: doc-docol:
! 1717: doc-docon:
! 1718: doc-dovar:
! 1719: doc-douser:
! 1720: doc-dodefer:
! 1721: doc-dofield:
! 1722:
! 1723: Currently there is no installation-independent way for recogizing words
! 1724: defined by a @code{CREATE}...@code{DOES>} word; however, once you know
! 1725: that a word is defined by a @code{CREATE}...@code{DOES>} word, you can
! 1726: use @code{>DOES-CODE}.
1.14 anton 1727:
1.4 anton 1728: @node ANS conformance, Model, Words, Top
1729: @chapter ANS conformance
1730:
1.17 anton 1731: To the best of our knowledge, Gforth is an
1.14 anton 1732:
1.15 anton 1733: ANS Forth System
1734: @itemize
1735: @item providing the Core Extensions word set
1736: @item providing the Block word set
1737: @item providing the Block Extensions word set
1738: @item providing the Double-Number word set
1739: @item providing the Double-Number Extensions word set
1740: @item providing the Exception word set
1741: @item providing the Exception Extensions word set
1742: @item providing the Facility word set
1743: @item providing @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
1744: @item providing the File Access word set
1745: @item providing the File Access Extensions word set
1746: @item providing the Floating-Point word set
1747: @item providing the Floating-Point Extensions word set
1748: @item providing the Locals word set
1749: @item providing the Locals Extensions word set
1750: @item providing the Memory-Allocation word set
1751: @item providing the Memory-Allocation Extensions word set (that one's easy)
1752: @item providing the Programming-Tools word set
1.18 ! anton 1753: @item providing @code{;code}, @code{AHEAD}, @code{ASSEMBLER}, @code{BYE}, @code{CODE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set
1.15 anton 1754: @item providing the Search-Order word set
1755: @item providing the Search-Order Extensions word set
1756: @item providing the String word set
1757: @item providing the String Extensions word set (another easy one)
1758: @end itemize
1759:
1760: In addition, ANS Forth systems are required to document certain
1761: implementation choices. This chapter tries to meet these
1762: requirements. In many cases it gives a way to ask the system for the
1763: information instead of providing the information directly, in
1764: particular, if the information depends on the processor, the operating
1765: system or the installation options chosen, or if they are likely to
1.17 anton 1766: change during the maintenance of Gforth.
1.15 anton 1767:
1.14 anton 1768: @comment The framework for the rest has been taken from pfe.
1769:
1770: @menu
1771: * The Core Words::
1772: * The optional Block word set::
1773: * The optional Double Number word set::
1774: * The optional Exception word set::
1775: * The optional Facility word set::
1776: * The optional File-Access word set::
1777: * The optional Floating-Point word set::
1778: * The optional Locals word set::
1779: * The optional Memory-Allocation word set::
1780: * The optional Programming-Tools word set::
1781: * The optional Search-Order word set::
1782: @end menu
1783:
1784:
1785: @c =====================================================================
1786: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
1787: @comment node-name, next, previous, up
1788: @section The Core Words
1789: @c =====================================================================
1790:
1791: @menu
1.15 anton 1792: * core-idef:: Implementation Defined Options
1793: * core-ambcond:: Ambiguous Conditions
1794: * core-other:: Other System Documentation
1.14 anton 1795: @end menu
1796:
1797: @c ---------------------------------------------------------------------
1798: @node core-idef, core-ambcond, The Core Words, The Core Words
1799: @subsection Implementation Defined Options
1800: @c ---------------------------------------------------------------------
1801:
1802: @table @i
1803:
1804: @item (Cell) aligned addresses:
1.17 anton 1805: processor-dependent. Gforth's alignment words perform natural alignment
1.14 anton 1806: (e.g., an address aligned for a datum of size 8 is divisible by
1807: 8). Unaligned accesses usually result in a @code{-23 THROW}.
1808:
1809: @item @code{EMIT} and non-graphic characters:
1810: The character is output using the C library function (actually, macro)
1811: @code{putchar}.
1812:
1813: @item character editing of @code{ACCEPT} and @code{EXPECT}:
1814: This is modeled on the GNU readline library (@pxref{Readline
1815: Interaction, , Command Line Editing, readline, The GNU Readline
1816: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
1817: producing a full word completion every time you type it (instead of
1818: producing the common prefix of all completions).
1819:
1820: @item character set:
1821: The character set of your computer and display device. Gforth is
1822: 8-bit-clean (but some other component in your system may make trouble).
1823:
1824: @item Character-aligned address requirements:
1825: installation-dependent. Currently a character is represented by a C
1826: @code{unsigned char}; in the future we might switch to @code{wchar_t}
1827: (Comments on that requested).
1828:
1829: @item character-set extensions and matching of names:
1.17 anton 1830: Any character except the ASCII NUL charcter can be used in a
1831: name. Matching is case-insensitive. The matching is performed using the
1832: C function @code{strncasecmp}, whose function is probably influenced by
1833: the locale. E.g., the @code{C} locale does not know about accents and
1.14 anton 1834: umlauts, so they are matched case-sensitively in that locale. For
1835: portability reasons it is best to write programs such that they work in
1836: the @code{C} locale. Then one can use libraries written by a Polish
1837: programmer (who might use words containing ISO Latin-2 encoded
1838: characters) and by a French programmer (ISO Latin-1) in the same program
1839: (of course, @code{WORDS} will produce funny results for some of the
1840: words (which ones, depends on the font you are using)). Also, the locale
1841: you prefer may not be available in other operating systems. Hopefully,
1842: Unicode will solve these problems one day.
1843:
1844: @item conditions under which control characters match a space delimiter:
1845: If @code{WORD} is called with the space character as a delimiter, all
1846: white-space characters (as identified by the C macro @code{isspace()})
1847: are delimiters. @code{PARSE}, on the other hand, treats space like other
1848: delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves
1849: like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer
1850: interpreter (aka text interpreter) by default, treats all white-space
1851: characters as delimiters.
1852:
1853: @item format of the control flow stack:
1854: The data stack is used as control flow stack. The size of a control flow
1855: stack item in cells is given by the constant @code{cs-item-size}. At the
1856: time of this writing, an item consists of a (pointer to a) locals list
1857: (third), an address in the code (second), and a tag for identifying the
1858: item (TOS). The following tags are used: @code{defstart},
1859: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
1860: @code{scopestart}.
1861:
1862: @item conversion of digits > 35
1863: The characters @code{[\]^_'} are the digits with the decimal value
1864: 36@minus{}41. There is no way to input many of the larger digits.
1865:
1866: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
1867: The cursor is moved to the end of the entered string. If the input is
1868: terminated using the @kbd{Return} key, a space is typed.
1869:
1870: @item exception abort sequence of @code{ABORT"}:
1871: The error string is stored into the variable @code{"error} and a
1872: @code{-2 throw} is performed.
1873:
1874: @item input line terminator:
1875: For interactive input, @kbd{C-m} and @kbd{C-j} terminate lines. One of
1876: these characters is typically produced when you type the @kbd{Enter} or
1877: @kbd{Return} key.
1878:
1879: @item maximum size of a counted string:
1880: @code{s" /counted-string" environment? drop .}. Currently 255 characters
1881: on all ports, but this may change.
1882:
1883: @item maximum size of a parsed string:
1884: Given by the constant @code{/line}. Currently 255 characters.
1885:
1886: @item maximum size of a definition name, in characters:
1887: 31
1888:
1889: @item maximum string length for @code{ENVIRONMENT?}, in characters:
1890: 31
1891:
1892: @item method of selecting the user input device:
1.17 anton 1893: The user input device is the standard input. There is currently no way to
1894: change it from within Gforth. However, the input can typically be
1895: redirected in the command line that starts Gforth.
1.14 anton 1896:
1897: @item method of selecting the user output device:
1898: The user output device is the standard output. It cannot be redirected
1.17 anton 1899: from within Gforth, but typically from the command line that starts
1900: Gforth. Gforth uses buffered output, so output on a terminal does not
1.14 anton 1901: become visible before the next newline or buffer overflow. Output on
1902: non-terminals is invisible until the buffer overflows.
1903:
1904: @item methods of dictionary compilation:
1.17 anton 1905: What are we expected to document here?
1.14 anton 1906:
1907: @item number of bits in one address unit:
1908: @code{s" address-units-bits" environment? drop .}. 8 in all current
1909: ports.
1910:
1911: @item number representation and arithmetic:
1912: Processor-dependent. Binary two's complement on all current ports.
1913:
1914: @item ranges for integer types:
1915: Installation-dependent. Make environmental queries for @code{MAX-N},
1916: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
1917: unsigned (and positive) types is 0. The lower bound for signed types on
1918: two's complement and one's complement machines machines can be computed
1919: by adding 1 to the upper bound.
1920:
1921: @item read-only data space regions:
1922: The whole Forth data space is writable.
1923:
1924: @item size of buffer at @code{WORD}:
1925: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1926: shared with the pictured numeric output string. If overwriting
1927: @code{PAD} is acceptable, it is as large as the remaining dictionary
1928: space, although only as much can be sensibly used as fits in a counted
1929: string.
1930:
1931: @item size of one cell in address units:
1932: @code{1 cells .}.
1933:
1934: @item size of one character in address units:
1935: @code{1 chars .}. 1 on all current ports.
1936:
1937: @item size of the keyboard terminal buffer:
1938: Varies. You can determine the size at a specific time using @code{lp@
1939: tib - .}. It is shared with the locals stack and TIBs of files that
1940: include the current file. You can change the amount of space for TIBs
1.17 anton 1941: and locals stack at Gforth startup with the command line option
1.14 anton 1942: @code{-l}.
1943:
1944: @item size of the pictured numeric output buffer:
1945: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1946: shared with @code{WORD}.
1947:
1948: @item size of the scratch area returned by @code{PAD}:
1949: The remainder of dictionary space. You can even use the unused part of
1950: the data stack space. The current size can be computed with @code{sp@
1951: pad - .}.
1952:
1953: @item system case-sensitivity characteristics:
1954: Dictionary searches are case insensitive. However, as explained above
1955: under @i{character-set extensions}, the matching for non-ASCII
1956: characters is determined by the locale you are using. In the default
1957: @code{C} locale all non-ASCII characters are matched case-sensitively.
1958:
1959: @item system prompt:
1960: @code{ ok} in interpret state, @code{ compiled} in compile state.
1961:
1962: @item division rounding:
1963: installation dependent. @code{s" floored" environment? drop .}. We leave
1964: the choice to gcc (what to use for @code{/}) and to you (whether to use
1965: @code{fm/mod}, @code{sm/rem} or simply @code{/}).
1966:
1967: @item values of @code{STATE} when true:
1968: -1.
1969:
1970: @item values returned after arithmetic overflow:
1971: On two's complement machines, arithmetic is performed modulo
1972: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1973: arithmetic (with appropriate mapping for signed types). Division by zero
1974: typically results in a @code{-55 throw} (floatingpoint unidentified
1975: fault), although a @code{-10 throw} (divide by zero) would be more
1976: appropriate.
1977:
1978: @item whether the current definition can be found after @t{DOES>}:
1979: No.
1980:
1981: @end table
1982:
1983: @c ---------------------------------------------------------------------
1984: @node core-ambcond, core-other, core-idef, The Core Words
1985: @subsection Ambiguous conditions
1986: @c ---------------------------------------------------------------------
1987:
1988: @table @i
1989:
1990: @item a name is neither a word nor a number:
1991: @code{-13 throw} (Undefined word)
1992:
1993: @item a definition name exceeds the maximum length allowed:
1994: @code{-19 throw} (Word name too long)
1995:
1996: @item addressing a region not inside the various data spaces of the forth system:
1997: The stacks, code space and name space are accessible. Machine code space is
1998: typically readable. Accessing other addresses gives results dependent on
1999: the operating system. On decent systems: @code{-9 throw} (Invalid memory
2000: address).
2001:
2002: @item argument type incompatible with parameter:
2003: This is usually not caught. Some words perform checks, e.g., the control
2004: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
2005: mismatch).
2006:
2007: @item attempting to obtain the execution token of a word with undefined execution semantics:
2008: You get an execution token representing the compilation semantics
2009: instead.
2010:
2011: @item dividing by zero:
2012: typically results in a @code{-55 throw} (floating point unidentified
2013: fault), although a @code{-10 throw} (divide by zero) would be more
2014: appropriate.
2015:
2016: @item insufficient data stack or return stack space:
2017: Not checked. This typically results in mysterious illegal memory
2018: accesses, producing @code{-9 throw} (Invalid memory address) or
2019: @code{-23 throw} (Address alignment exception).
2020:
2021: @item insufficient space for loop control parameters:
2022: like other return stack overflows.
2023:
2024: @item insufficient space in the dictionary:
2025: Not checked. Similar results as stack overflows. However, typically the
2026: error appears at a different place when one inserts or removes code.
2027:
2028: @item interpreting a word with undefined interpretation semantics:
2029: For some words, we defined interpretation semantics. For the others:
2030: @code{-14 throw} (Interpreting a compile-only word). Note that this is
2031: checked only by the outer (aka text) interpreter; if the word is
2032: @code{execute}d in some other way, it will typically perform it's
2033: compilation semantics even in interpret state. (We could change @code{'}
2034: and relatives not to give the xt of such words, but we think that would
2035: be too restrictive).
2036:
2037: @item modifying the contents of the input buffer or a string literal:
2038: These are located in writable memory and can be modified.
2039:
2040: @item overflow of the pictured numeric output string:
2041: Not checked.
2042:
2043: @item parsed string overflow:
2044: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
2045:
2046: @item producing a result out of range:
2047: On two's complement machines, arithmetic is performed modulo
2048: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
2049: arithmetic (with appropriate mapping for signed types). Division by zero
2050: typically results in a @code{-55 throw} (floatingpoint unidentified
2051: fault), although a @code{-10 throw} (divide by zero) would be more
2052: appropriate. @code{convert} and @code{>number} currently overflow
2053: silently.
2054:
2055: @item reading from an empty data or return stack:
2056: The data stack is checked by the outer (aka text) interpreter after
2057: every word executed. If it has underflowed, a @code{-4 throw} (Stack
2058: underflow) is performed. Apart from that, the stacks are not checked and
2059: underflows can result in similar behaviour as overflows (of adjacent
2060: stacks).
2061:
2062: @item unexepected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
2063: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
2064: use zero-length string as a name). Words like @code{'} probably will not
2065: find what they search. Note that it is possible to create zero-length
2066: names with @code{nextname} (should it not?).
2067:
2068: @item @code{>IN} greater than input buffer:
2069: The next invocation of a parsing word returns a string wih length 0.
2070:
2071: @item @code{RECURSE} appears after @code{DOES>}:
2072: Compiles a recursive call to the defining word not to the defined word.
2073:
2074: @item argument input source different than current input source for @code{RESTORE-INPUT}:
2075: !!???If the argument input source is a valid input source then it gets
2076: restored. Otherwise causes @code{-12 THROW} which unless caught issues
2077: the message "argument type mismatch" and aborts.
2078:
2079: @item data space containing definitions gets de-allocated:
2080: Deallocation with @code{allot} is not checked. This typically resuls in
2081: memory access faults or execution of illegal instructions.
2082:
2083: @item data space read/write with incorrect alignment:
2084: Processor-dependent. Typically results in a @code{-23 throw} (Address
2085: alignment exception). Under Linux on a 486 or later processor with
2086: alignment turned on, incorrect alignment results in a @code{-9 throw}
2087: (Invalid memory address). There are reportedly some processors with
2088: alignment restrictions that do not report them.
2089:
2090: @item data space pointer not properly aligned, @code{,}, @code{C,}:
2091: Like other alignment errors.
2092:
2093: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
2094: Not checked. May cause an illegal memory access.
2095:
2096: @item loop control parameters not available:
2097: Not checked. The counted loop words simply assume that the top of return
2098: stack items are loop control parameters and behave accordingly.
2099:
2100: @item most recent definition does not have a name (@code{IMMEDIATE}):
2101: @code{abort" last word was headerless"}.
2102:
2103: @item name not defined by @code{VALUE} used by @code{TO}:
2104: @code{-32 throw} (Invalid name argument)
2105:
1.15 anton 2106: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
1.14 anton 2107: @code{-13 throw} (Undefined word)
2108:
2109: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
2110: Gforth behaves as if they were of the same type. I.e., you can predict
2111: the behaviour by interpreting all parameters as, e.g., signed.
2112:
2113: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
2114: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} is equivalent to
2115: @code{TO}.
2116:
2117: @item String longer than a counted string returned by @code{WORD}:
2118: Not checked. The string will be ok, but the count will, of course,
2119: contain only the least significant bits of the length.
2120:
1.15 anton 2121: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
1.14 anton 2122: Processor-dependent. Typical behaviours are returning 0 and using only
2123: the low bits of the shift count.
2124:
2125: @item word not defined via @code{CREATE}:
2126: @code{>BODY} produces the PFA of the word no matter how it was defined.
2127:
2128: @code{DOES>} changes the execution semantics of the last defined word no
2129: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
2130: @code{CREATE , DOES>}.
2131:
2132: @item words improperly used outside @code{<#} and @code{#>}:
2133: Not checked. As usual, you can expect memory faults.
2134:
2135: @end table
2136:
2137:
2138: @c ---------------------------------------------------------------------
2139: @node core-other, , core-ambcond, The Core Words
2140: @subsection Other system documentation
2141: @c ---------------------------------------------------------------------
2142:
2143: @table @i
2144:
2145: @item nonstandard words using @code{PAD}:
2146: None.
2147:
2148: @item operator's terminal facilities available:
2149: !!??
2150:
2151: @item program data space available:
2152: @code{sp@ here - .} gives the space remaining for dictionary and data
2153: stack together.
2154:
2155: @item return stack space available:
2156: !!??
2157:
2158: @item stack space available:
2159: @code{sp@ here - .} gives the space remaining for dictionary and data
2160: stack together.
2161:
2162: @item system dictionary space required, in address units:
2163: Type @code{here forthstart - .} after startup. At the time of this
2164: writing, this gives 70108 (bytes) on a 32-bit system.
2165: @end table
2166:
2167:
2168: @c =====================================================================
2169: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
2170: @section The optional Block word set
2171: @c =====================================================================
2172:
2173: @menu
1.15 anton 2174: * block-idef:: Implementation Defined Options
2175: * block-ambcond:: Ambiguous Conditions
2176: * block-other:: Other System Documentation
1.14 anton 2177: @end menu
2178:
2179:
2180: @c ---------------------------------------------------------------------
2181: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
2182: @subsection Implementation Defined Options
2183: @c ---------------------------------------------------------------------
2184:
2185: @table @i
2186:
2187: @item the format for display by @code{LIST}:
2188: First the screen number is displayed, then 16 lines of 64 characters,
2189: each line preceded by the line number.
2190:
2191: @item the length of a line affected by @code{\}:
2192: 64 characters.
2193: @end table
2194:
2195:
2196: @c ---------------------------------------------------------------------
2197: @node block-ambcond, block-other, block-idef, The optional Block word set
2198: @subsection Ambiguous conditions
2199: @c ---------------------------------------------------------------------
2200:
2201: @table @i
2202:
2203: @item correct block read was not possible:
2204: Typically results in a @code{throw} of some OS-derived value (between
2205: -512 and -2048). If the blocks file was just not long enough, blanks are
2206: supplied for the missing portion.
2207:
2208: @item I/O exception in block transfer:
2209: Typically results in a @code{throw} of some OS-derived value (between
2210: -512 and -2048).
2211:
2212: @item invalid block number:
2213: @code{-35 throw} (Invalid block number)
2214:
2215: @item a program directly alters the contents of @code{BLK}:
2216: The input stream is switched to that other block, at the same
2217: position. If the storing to @code{BLK} happens when interpreting
2218: non-block input, the system will get quite confused when the block ends.
2219:
2220: @item no current block buffer for @code{UPDATE}:
2221: @code{UPDATE} has no effect.
2222:
2223: @end table
2224:
2225:
2226: @c ---------------------------------------------------------------------
2227: @node block-other, , block-ambcond, The optional Block word set
2228: @subsection Other system documentation
2229: @c ---------------------------------------------------------------------
2230:
2231: @table @i
2232:
2233: @item any restrictions a multiprogramming system places on the use of buffer addresses:
2234: No restrictions (yet).
2235:
2236: @item the number of blocks available for source and data:
2237: depends on your disk space.
2238:
2239: @end table
2240:
2241:
2242: @c =====================================================================
2243: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
2244: @section The optional Double Number word set
2245: @c =====================================================================
2246:
2247: @menu
1.15 anton 2248: * double-ambcond:: Ambiguous Conditions
1.14 anton 2249: @end menu
2250:
2251:
2252: @c ---------------------------------------------------------------------
1.15 anton 2253: @node double-ambcond, , The optional Double Number word set, The optional Double Number word set
1.14 anton 2254: @subsection Ambiguous conditions
2255: @c ---------------------------------------------------------------------
2256:
2257: @table @i
2258:
1.15 anton 2259: @item @var{d} outside of range of @var{n} in @code{D>S}:
1.14 anton 2260: The least significant cell of @var{d} is produced.
2261:
2262: @end table
2263:
2264:
2265: @c =====================================================================
2266: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
2267: @section The optional Exception word set
2268: @c =====================================================================
2269:
2270: @menu
1.15 anton 2271: * exception-idef:: Implementation Defined Options
1.14 anton 2272: @end menu
2273:
2274:
2275: @c ---------------------------------------------------------------------
1.15 anton 2276: @node exception-idef, , The optional Exception word set, The optional Exception word set
1.14 anton 2277: @subsection Implementation Defined Options
2278: @c ---------------------------------------------------------------------
2279:
2280: @table @i
2281: @item @code{THROW}-codes used in the system:
2282: The codes -256@minus{}-511 are used for reporting signals (see
2283: @file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors
2284: (for file and memory allocation operations). The mapping from OS error
2285: numbers to throw code is -512@minus{}@var{errno}. One side effect of
2286: this mapping is that undefined OS errors produce a message with a
2287: strange number; e.g., @code{-1000 THROW} results in @code{Unknown error
2288: 488} on my system.
2289: @end table
2290:
2291: @c =====================================================================
2292: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
2293: @section The optional Facility word set
2294: @c =====================================================================
2295:
2296: @menu
1.15 anton 2297: * facility-idef:: Implementation Defined Options
2298: * facility-ambcond:: Ambiguous Conditions
1.14 anton 2299: @end menu
2300:
2301:
2302: @c ---------------------------------------------------------------------
2303: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
2304: @subsection Implementation Defined Options
2305: @c ---------------------------------------------------------------------
2306:
2307: @table @i
2308:
2309: @item encoding of keyboard events (@code{EKEY}):
2310: Not yet implemeted.
2311:
2312: @item duration of a system clock tick
2313: System dependent. With respect to @code{MS}, the time is specified in
2314: microseconds. How well the OS and the hardware implement this, is
2315: another question.
2316:
2317: @item repeatability to be expected from the execution of @code{MS}:
2318: System dependent. On Unix, a lot depends on load. If the system is
1.17 anton 2319: lightly loaded, and the delay is short enough that Gforth does not get
1.14 anton 2320: swapped out, the performance should be acceptable. Under MS-DOS and
2321: other single-tasking systems, it should be good.
2322:
2323: @end table
2324:
2325:
2326: @c ---------------------------------------------------------------------
1.15 anton 2327: @node facility-ambcond, , facility-idef, The optional Facility word set
1.14 anton 2328: @subsection Ambiguous conditions
2329: @c ---------------------------------------------------------------------
2330:
2331: @table @i
2332:
2333: @item @code{AT-XY} can't be performed on user output device:
2334: Largely terminal dependant. No range checks are done on the arguments.
2335: No errors are reported. You may see some garbage appearing, you may see
2336: simply nothing happen.
2337:
2338: @end table
2339:
2340:
2341: @c =====================================================================
2342: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
2343: @section The optional File-Access word set
2344: @c =====================================================================
2345:
2346: @menu
1.15 anton 2347: * file-idef:: Implementation Defined Options
2348: * file-ambcond:: Ambiguous Conditions
1.14 anton 2349: @end menu
2350:
2351:
2352: @c ---------------------------------------------------------------------
2353: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
2354: @subsection Implementation Defined Options
2355: @c ---------------------------------------------------------------------
2356:
2357: @table @i
2358:
2359: @item File access methods used:
2360: @code{R/O}, @code{R/W} and @code{BIN} work as you would
2361: expect. @code{W/O} translates into the C file opening mode @code{w} (or
2362: @code{wb}): The file is cleared, if it exists, and created, if it does
1.15 anton 2363: not (both with @code{open-file} and @code{create-file}). Under Unix
1.14 anton 2364: @code{create-file} creates a file with 666 permissions modified by your
2365: umask.
2366:
2367: @item file exceptions:
2368: The file words do not raise exceptions (except, perhaps, memory access
2369: faults when you pass illegal addresses or file-ids).
2370:
2371: @item file line terminator:
2372: System-dependent. Gforth uses C's newline character as line
2373: terminator. What the actual character code(s) of this are is
2374: system-dependent.
2375:
2376: @item file name format
2377: System dependent. Gforth just uses the file name format of your OS.
2378:
2379: @item information returned by @code{FILE-STATUS}:
2380: @code{FILE-STATUS} returns the most powerful file access mode allowed
2381: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
2382: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
2383: along with the retured mode.
2384:
2385: @item input file state after an exception when including source:
2386: All files that are left via the exception are closed.
2387:
2388: @item @var{ior} values and meaning:
1.15 anton 2389: The @var{ior}s returned by the file and memory allocation words are
2390: intended as throw codes. They typically are in the range
2391: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2392: @var{ior}s is -512@minus{}@var{errno}.
1.14 anton 2393:
2394: @item maximum depth of file input nesting:
2395: limited by the amount of return stack, locals/TIB stack, and the number
2396: of open files available. This should not give you troubles.
2397:
2398: @item maximum size of input line:
2399: @code{/line}. Currently 255.
2400:
2401: @item methods of mapping block ranges to files:
2402: Currently, the block words automatically access the file
2403: @file{blocks.fb} in the currend working directory. More sophisticated
2404: methods could be implemented if there is demand (and a volunteer).
2405:
2406: @item number of string buffers provided by @code{S"}:
2407: 1
2408:
2409: @item size of string buffer used by @code{S"}:
2410: @code{/line}. currently 255.
2411:
2412: @end table
2413:
2414: @c ---------------------------------------------------------------------
1.15 anton 2415: @node file-ambcond, , file-idef, The optional File-Access word set
1.14 anton 2416: @subsection Ambiguous conditions
2417: @c ---------------------------------------------------------------------
2418:
2419: @table @i
2420:
2421: @item attempting to position a file outside it's boundaries:
2422: @code{REPOSITION-FILE} is performed as usual: Afterwards,
2423: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
2424:
2425: @item attempting to read from file positions not yet written:
2426: End-of-file, i.e., zero characters are read and no error is reported.
2427:
2428: @item @var{file-id} is invalid (@code{INCLUDE-FILE}):
2429: An appropriate exception may be thrown, but a memory fault or other
2430: problem is more probable.
2431:
2432: @item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}):
2433: The @var{ior} produced by the operation, that discovered the problem, is
2434: thrown.
2435:
2436: @item named file cannot be opened (@code{included}):
2437: The @var{ior} produced by @code{open-file} is thrown.
2438:
2439: @item requesting an unmapped block number:
2440: There are no unmapped legal block numbers. On some operating systems,
2441: writing a block with a large number may overflow the file system and
2442: have an error message as consequence.
2443:
2444: @item using @code{source-id} when @code{blk} is non-zero:
2445: @code{source-id} performs its function. Typically it will give the id of
2446: the source which loaded the block. (Better ideas?)
2447:
2448: @end table
2449:
2450:
2451: @c =====================================================================
2452: @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
1.15 anton 2453: @section The optional Floating-Point word set
1.14 anton 2454: @c =====================================================================
2455:
2456: @menu
1.15 anton 2457: * floating-idef:: Implementation Defined Options
2458: * floating-ambcond:: Ambiguous Conditions
1.14 anton 2459: @end menu
2460:
2461:
2462: @c ---------------------------------------------------------------------
2463: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
2464: @subsection Implementation Defined Options
2465: @c ---------------------------------------------------------------------
2466:
2467: @table @i
2468:
1.15 anton 2469: @item format and range of floating point numbers:
2470: System-dependent; the @code{double} type of C.
1.14 anton 2471:
1.15 anton 2472: @item results of @code{REPRESENT} when @var{float} is out of range:
2473: System dependent; @code{REPRESENT} is implemented using the C library
2474: function @code{ecvt()} and inherits its behaviour in this respect.
1.14 anton 2475:
1.15 anton 2476: @item rounding or truncation of floating-point numbers:
2477: What's the question?!!
1.14 anton 2478:
1.15 anton 2479: @item size of floating-point stack:
2480: @code{s" FLOATING-STACK" environment? drop .}. Can be changed at startup
2481: with the command-line option @code{-f}.
1.14 anton 2482:
1.15 anton 2483: @item width of floating-point stack:
2484: @code{1 floats}.
1.14 anton 2485:
2486: @end table
2487:
2488:
2489: @c ---------------------------------------------------------------------
1.15 anton 2490: @node floating-ambcond, , floating-idef, The optional Floating-Point word set
2491: @subsection Ambiguous conditions
1.14 anton 2492: @c ---------------------------------------------------------------------
2493:
2494: @table @i
2495:
1.15 anton 2496: @item @code{df@@} or @code{df!} used with an address that is not double-float aligned:
2497: System-dependent. Typically results in an alignment fault like other
2498: alignment violations.
1.14 anton 2499:
1.15 anton 2500: @item @code{f@@} or @code{f!} used with an address that is not float aligned:
2501: System-dependent. Typically results in an alignment fault like other
2502: alignment violations.
1.14 anton 2503:
1.15 anton 2504: @item Floating-point result out of range:
2505: System-dependent. Can result in a @code{-55 THROW} (Floating-point
2506: unidentified fault), or can produce a special value representing, e.g.,
2507: Infinity.
1.14 anton 2508:
1.15 anton 2509: @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned:
2510: System-dependent. Typically results in an alignment fault like other
2511: alignment violations.
1.14 anton 2512:
1.15 anton 2513: @item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
2514: The floating-point number is converted into decimal nonetheless.
1.14 anton 2515:
1.15 anton 2516: @item Both arguments are equal to zero (@code{FATAN2}):
2517: System-dependent. @code{FATAN2} is implemented using the C library
2518: function @code{atan2()}.
1.14 anton 2519:
1.15 anton 2520: @item Using ftan on an argument @var{r1} where cos(@var{r1}) is zero:
2521: System-dependent. Anyway, typically the cos of @var{r1} will not be zero
2522: because of small errors and the tan will be a very large (or very small)
2523: but finite number.
1.14 anton 2524:
1.15 anton 2525: @item @var{d} cannot be presented precisely as a float in @code{D>F}:
2526: The result is rounded to the nearest float.
1.14 anton 2527:
1.15 anton 2528: @item dividing by zero:
2529: @code{-55 throw} (Floating-point unidentified fault)
1.14 anton 2530:
1.15 anton 2531: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
2532: System dependent. On IEEE-FP based systems the number is converted into
2533: an infinity.
1.14 anton 2534:
1.15 anton 2535: @item @var{float}<1 (@code{facosh}):
2536: @code{-55 throw} (Floating-point unidentified fault)
1.14 anton 2537:
1.15 anton 2538: @item @var{float}=<-1 (@code{flnp1}):
2539: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2540: negative infinity is typically produced for @var{float}=-1.
1.14 anton 2541:
1.15 anton 2542: @item @var{float}=<0 (@code{fln}, @code{flog}):
2543: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2544: negative infinity is typically produced for @var{float}=0.
1.14 anton 2545:
1.15 anton 2546: @item @var{float}<0 (@code{fasinh}, @code{fsqrt}):
2547: @code{-55 throw} (Floating-point unidentified fault). @code{fasinh}
2548: produces values for these inputs on my Linux box (Bug in the C library?)
1.14 anton 2549:
1.15 anton 2550: @item |@var{float}|>1 (@code{facos}, @code{fasin}, @code{fatanh}):
2551: @code{-55 throw} (Floating-point unidentified fault).
1.14 anton 2552:
1.15 anton 2553: @item integer part of float cannot be represented by @var{d} in @code{f>d}:
2554: @code{-55 throw} (Floating-point unidentified fault).
1.14 anton 2555:
1.15 anton 2556: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
2557: This does not happen.
2558: @end table
1.14 anton 2559:
2560:
2561:
2562: @c =====================================================================
1.15 anton 2563: @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
2564: @section The optional Locals word set
1.14 anton 2565: @c =====================================================================
2566:
2567: @menu
1.15 anton 2568: * locals-idef:: Implementation Defined Options
2569: * locals-ambcond:: Ambiguous Conditions
1.14 anton 2570: @end menu
2571:
2572:
2573: @c ---------------------------------------------------------------------
1.15 anton 2574: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
1.14 anton 2575: @subsection Implementation Defined Options
2576: @c ---------------------------------------------------------------------
2577:
2578: @table @i
2579:
1.15 anton 2580: @item maximum number of locals in a definition:
2581: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
2582: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
2583: characters. The number of locals in a definition is bounded by the size
2584: of locals-buffer, which contains the names of the locals.
1.14 anton 2585:
2586: @end table
2587:
2588:
2589: @c ---------------------------------------------------------------------
1.15 anton 2590: @node locals-ambcond, , locals-idef, The optional Locals word set
1.14 anton 2591: @subsection Ambiguous conditions
2592: @c ---------------------------------------------------------------------
2593:
2594: @table @i
2595:
1.15 anton 2596: @item executing a named local in interpretation state:
2597: @code{-14 throw} (Interpreting a compile-only word).
1.14 anton 2598:
1.15 anton 2599: @item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
2600: @code{-32 throw} (Invalid name argument)
1.14 anton 2601:
2602: @end table
2603:
2604:
2605: @c =====================================================================
1.15 anton 2606: @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
2607: @section The optional Memory-Allocation word set
1.14 anton 2608: @c =====================================================================
2609:
2610: @menu
1.15 anton 2611: * memory-idef:: Implementation Defined Options
1.14 anton 2612: @end menu
2613:
2614:
2615: @c ---------------------------------------------------------------------
1.15 anton 2616: @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
1.14 anton 2617: @subsection Implementation Defined Options
2618: @c ---------------------------------------------------------------------
2619:
2620: @table @i
2621:
1.15 anton 2622: @item values and meaning of @var{ior}:
2623: The @var{ior}s returned by the file and memory allocation words are
2624: intended as throw codes. They typically are in the range
2625: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2626: @var{ior}s is -512@minus{}@var{errno}.
1.14 anton 2627:
2628: @end table
2629:
2630: @c =====================================================================
1.15 anton 2631: @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
2632: @section The optional Programming-Tools word set
1.14 anton 2633: @c =====================================================================
2634:
2635: @menu
1.15 anton 2636: * programming-idef:: Implementation Defined Options
2637: * programming-ambcond:: Ambiguous Conditions
1.14 anton 2638: @end menu
2639:
2640:
2641: @c ---------------------------------------------------------------------
1.15 anton 2642: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
1.14 anton 2643: @subsection Implementation Defined Options
2644: @c ---------------------------------------------------------------------
2645:
2646: @table @i
2647:
1.15 anton 2648: @item ending sequence for input following @code{;code} and @code{code}:
2649: Not implemented (yet).
1.14 anton 2650:
1.15 anton 2651: @item manner of processing input following @code{;code} and @code{code}:
2652: Not implemented (yet).
2653:
2654: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
2655: Not implemented (yet). If they were implemented, they would use the
2656: search order wordset.
2657:
2658: @item source and format of display by @code{SEE}:
2659: The source for @code{see} is the intermediate code used by the inner
2660: interpreter. The current @code{see} tries to output Forth source code
2661: as well as possible.
2662:
1.14 anton 2663: @end table
2664:
2665: @c ---------------------------------------------------------------------
1.15 anton 2666: @node programming-ambcond, , programming-idef, The optional Programming-Tools word set
1.14 anton 2667: @subsection Ambiguous conditions
2668: @c ---------------------------------------------------------------------
2669:
2670: @table @i
2671:
1.15 anton 2672: @item deleting the compilation wordlist (@code{FORGET}):
2673: Not implemented (yet).
1.14 anton 2674:
1.15 anton 2675: @item fewer than @var{u}+1 items on the control flow stack (@code{CS-PICK}, @code{CS-ROLL}):
2676: This typically results in an @code{abort"} with a descriptive error
2677: message (may change into a @code{-22 throw} (Control structure mismatch)
2678: in the future). You may also get a memory access error. If you are
2679: unlucky, this ambiguous condition is not caught.
2680:
2681: @item @var{name} can't be found (@code{forget}):
2682: Not implemented (yet).
1.14 anton 2683:
1.15 anton 2684: @item @var{name} not defined via @code{CREATE}:
2685: @code{;code} is not implemented (yet). If it were, it would behave like
2686: @code{DOES>} in this respect, i.e., change the execution semantics of
2687: the last defined word no matter how it was defined.
1.14 anton 2688:
1.15 anton 2689: @item @code{POSTPONE} applied to @code{[IF]}:
2690: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
2691: equivalent to @code{[IF]}.
1.14 anton 2692:
1.15 anton 2693: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
2694: Continue in the same state of conditional compilation in the next outer
2695: input source. Currently there is no warning to the user about this.
1.14 anton 2696:
1.15 anton 2697: @item removing a needed definition (@code{FORGET}):
2698: Not implemented (yet).
1.14 anton 2699:
2700: @end table
2701:
2702:
2703: @c =====================================================================
1.15 anton 2704: @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
2705: @section The optional Search-Order word set
1.14 anton 2706: @c =====================================================================
2707:
2708: @menu
1.15 anton 2709: * search-idef:: Implementation Defined Options
2710: * search-ambcond:: Ambiguous Conditions
1.14 anton 2711: @end menu
2712:
2713:
2714: @c ---------------------------------------------------------------------
1.15 anton 2715: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
1.14 anton 2716: @subsection Implementation Defined Options
2717: @c ---------------------------------------------------------------------
2718:
2719: @table @i
2720:
1.15 anton 2721: @item maximum number of word lists in search order:
2722: @code{s" wordlists" environment? drop .}. Currently 16.
2723:
2724: @item minimum search order:
2725: @code{root root}.
1.14 anton 2726:
2727: @end table
2728:
2729: @c ---------------------------------------------------------------------
1.15 anton 2730: @node search-ambcond, , search-idef, The optional Search-Order word set
1.14 anton 2731: @subsection Ambiguous conditions
2732: @c ---------------------------------------------------------------------
2733:
2734: @table @i
2735:
1.15 anton 2736: @item changing the compilation wordlist (during compilation):
2737: The definition is put into the wordlist that is the compilation wordlist
2738: when @code{REVEAL} is executed (by @code{;}, @code{DOES>},
2739: @code{RECURSIVE}, etc.).
1.14 anton 2740:
1.15 anton 2741: @item search order empty (@code{previous}):
2742: @code{abort" Vocstack empty"}.
1.14 anton 2743:
1.15 anton 2744: @item too many word lists in search order (@code{also}):
2745: @code{abort" Vocstack full"}.
1.14 anton 2746:
2747: @end table
1.13 anton 2748:
2749:
1.17 anton 2750: @node Model, Emacs and Gforth, ANS conformance, Top
1.4 anton 2751: @chapter Model
2752:
1.17 anton 2753: @node Emacs and Gforth, Internals, Model, Top
2754: @chapter Emacs and Gforth
1.4 anton 2755:
1.17 anton 2756: Gforth comes with @file{gforth.el}, an improved version of
1.4 anton 2757: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
2758: improvements are a better (but still not perfect) handling of
2759: indentation. I have also added comment paragraph filling (@kbd{M-q}),
1.8 anton 2760: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
2761: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
2762: stuff I do not use alone, even though some of it only makes sense for
2763: TILE. To get a description of these features, enter Forth mode and type
2764: @kbd{C-h m}.
1.4 anton 2765:
1.17 anton 2766: In addition, Gforth supports Emacs quite well: The source code locations
1.4 anton 2767: given in error messages, debugging output (from @code{~~}) and failed
2768: assertion messages are in the right format for Emacs' compilation mode
2769: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
2770: Manual}) so the source location corresponding to an error or other
2771: message is only a few keystrokes away (@kbd{C-x `} for the next error,
2772: @kbd{C-c C-c} for the error under the cursor).
2773:
2774: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
2775: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
2776: contains the definitions of all words defined afterwards. You can then
2777: find the source for a word using @kbd{M-.}. Note that emacs can use
1.17 anton 2778: several tags files at the same time (e.g., one for the Gforth sources
1.4 anton 2779: and one for your program).
2780:
2781: To get all these benefits, add the following lines to your @file{.emacs}
2782: file:
2783:
2784: @example
2785: (autoload 'forth-mode "gforth.el")
2786: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
2787: @end example
2788:
1.17 anton 2789: @node Internals, Bugs, Emacs and Gforth, Top
1.3 anton 2790: @chapter Internals
2791:
1.17 anton 2792: Reading this section is not necessary for programming with Gforth. It
2793: should be helpful for finding your way in the Gforth sources.
1.3 anton 2794:
1.4 anton 2795: @menu
2796: * Portability::
2797: * Threading::
2798: * Primitives::
2799: * System Architecture::
1.17 anton 2800: * Performance::
1.4 anton 2801: @end menu
2802:
2803: @node Portability, Threading, Internals, Internals
1.3 anton 2804: @section Portability
2805:
2806: One of the main goals of the effort is availability across a wide range
2807: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
2808: this goal by manually coding the engine in assembly language for several
2809: then-popular processors. This approach is very labor-intensive and the
2810: results are short-lived due to progress in computer architecture.
2811:
2812: Others have avoided this problem by coding in C, e.g., Mitch Bradley
2813: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
2814: particularly popular for UNIX-based Forths due to the large variety of
2815: architectures of UNIX machines. Unfortunately an implementation in C
2816: does not mix well with the goals of efficiency and with using
2817: traditional techniques: Indirect or direct threading cannot be expressed
2818: in C, and switch threading, the fastest technique available in C, is
2819: significantly slower. Another problem with C is that it's very
2820: cumbersome to express double integer arithmetic.
2821:
2822: Fortunately, there is a portable language that does not have these
2823: limitations: GNU C, the version of C processed by the GNU C compiler
2824: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
2825: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
2826: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
2827: threading possible, its @code{long long} type (@pxref{Long Long, ,
2828: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
2829: double numbers. GNU C is available for free on all important (and many
2830: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
2831: the Atari ST, so a Forth written in GNU C can run on all these
1.17 anton 2832: machines.
1.3 anton 2833:
2834: Writing in a portable language has the reputation of producing code that
2835: is slower than assembly. For our Forth engine we repeatedly looked at
2836: the code produced by the compiler and eliminated most compiler-induced
2837: inefficiencies by appropriate changes in the source-code.
2838:
2839: However, register allocation cannot be portably influenced by the
2840: programmer, leading to some inefficiencies on register-starved
2841: machines. We use explicit register declarations (@pxref{Explicit Reg
2842: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
2843: improve the speed on some machines. They are turned on by using the
2844: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
2845: only depends on the machine, but also on the compiler version: On some
2846: machines some compiler versions produce incorrect code when certain
2847: explicit register declarations are used. So by default
2848: @code{-DFORCE_REG} is not used.
2849:
1.4 anton 2850: @node Threading, Primitives, Portability, Internals
1.3 anton 2851: @section Threading
2852:
2853: GNU C's labels as values extension (available since @code{gcc-2.0},
2854: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
2855: makes it possible to take the address of @var{label} by writing
2856: @code{&&@var{label}}. This address can then be used in a statement like
2857: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
2858: @code{goto x}.
2859:
2860: With this feature an indirect threaded NEXT looks like:
2861: @example
2862: cfa = *ip++;
2863: ca = *cfa;
2864: goto *ca;
2865: @end example
2866: For those unfamiliar with the names: @code{ip} is the Forth instruction
2867: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
2868: execution token and points to the code field of the next word to be
2869: executed; The @code{ca} (code address) fetched from there points to some
2870: executable code, e.g., a primitive or the colon definition handler
2871: @code{docol}.
2872:
2873: Direct threading is even simpler:
2874: @example
2875: ca = *ip++;
2876: goto *ca;
2877: @end example
2878:
2879: Of course we have packaged the whole thing neatly in macros called
2880: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
2881:
1.4 anton 2882: @menu
2883: * Scheduling::
2884: * Direct or Indirect Threaded?::
2885: * DOES>::
2886: @end menu
2887:
2888: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1.3 anton 2889: @subsection Scheduling
2890:
2891: There is a little complication: Pipelined and superscalar processors,
2892: i.e., RISC and some modern CISC machines can process independent
2893: instructions while waiting for the results of an instruction. The
2894: compiler usually reorders (schedules) the instructions in a way that
2895: achieves good usage of these delay slots. However, on our first tries
2896: the compiler did not do well on scheduling primitives. E.g., for
2897: @code{+} implemented as
2898: @example
2899: n=sp[0]+sp[1];
2900: sp++;
2901: sp[0]=n;
2902: NEXT;
2903: @end example
2904: the NEXT comes strictly after the other code, i.e., there is nearly no
2905: scheduling. After a little thought the problem becomes clear: The
2906: compiler cannot know that sp and ip point to different addresses (and
1.4 anton 2907: the version of @code{gcc} we used would not know it even if it was
2908: possible), so it could not move the load of the cfa above the store to
2909: the TOS. Indeed the pointers could be the same, if code on or very near
2910: the top of stack were executed. In the interest of speed we chose to
2911: forbid this probably unused ``feature'' and helped the compiler in
2912: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
2913: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1.3 anton 2914: @example
2915: n=sp[0]+sp[1];
2916: sp++;
2917: NEXT_P1;
2918: sp[0]=n;
2919: NEXT_P2;
2920: @end example
1.4 anton 2921: This can be scheduled optimally by the compiler.
1.3 anton 2922:
2923: This division can be turned off with the switch @code{-DCISC_NEXT}. This
2924: switch is on by default on machines that do not profit from scheduling
2925: (e.g., the 80386), in order to preserve registers.
2926:
1.4 anton 2927: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1.3 anton 2928: @subsection Direct or Indirect Threaded?
2929:
2930: Both! After packaging the nasty details in macro definitions we
2931: realized that we could switch between direct and indirect threading by
2932: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
2933: defining a few machine-specific macros for the direct-threading case.
2934: On the Forth level we also offer access words that hide the
2935: differences between the threading methods (@pxref{Threading Words}).
2936:
2937: Indirect threading is implemented completely
2938: machine-independently. Direct threading needs routines for creating
2939: jumps to the executable code (e.g. to docol or dodoes). These routines
2940: are inherently machine-dependent, but they do not amount to many source
2941: lines. I.e., even porting direct threading to a new machine is a small
2942: effort.
2943:
1.4 anton 2944: @node DOES>, , Direct or Indirect Threaded?, Threading
1.3 anton 2945: @subsection DOES>
2946: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
2947: the chunk of code executed by every word defined by a
2948: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
2949: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
2950: DOES-code)? There are two solutions:
2951:
2952: In fig-Forth the code field points directly to the dodoes and the
2953: DOES-code address is stored in the cell after the code address
2954: (i.e. at cfa cell+). It may seem that this solution is illegal in the
2955: Forth-79 and all later standards, because in fig-Forth this address
2956: lies in the body (which is illegal in these standards). However, by
2957: making the code field larger for all words this solution becomes legal
2958: again. We use this approach for the indirect threaded version. Leaving
2959: a cell unused in most words is a bit wasteful, but on the machines we
2960: are targetting this is hardly a problem. The other reason for having a
2961: code field size of two cells is to avoid having different image files
1.4 anton 2962: for direct and indirect threaded systems (@pxref{System Architecture}).
1.3 anton 2963:
2964: The other approach is that the code field points or jumps to the cell
2965: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
2966: this address. @code{dodoes} can then get the DOES-code address by
2967: computing the code address, i.e., the address of the jump to dodoes,
2968: and add the length of that jump field. A variant of this is to have a
2969: call to @code{dodoes} after the @code{DOES>}; then the return address
2970: (which can be found in the return register on RISCs) is the DOES-code
2971: address. Since the two cells available in the code field are usually
2972: used up by the jump to the code address in direct threading, we use
2973: this approach for direct threading. We did not want to add another
2974: cell to the code field.
2975:
1.4 anton 2976: @node Primitives, System Architecture, Threading, Internals
1.3 anton 2977: @section Primitives
2978:
1.4 anton 2979: @menu
2980: * Automatic Generation::
2981: * TOS Optimization::
2982: * Produced code::
2983: @end menu
2984:
2985: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1.3 anton 2986: @subsection Automatic Generation
2987:
2988: Since the primitives are implemented in a portable language, there is no
2989: longer any need to minimize the number of primitives. On the contrary,
2990: having many primitives is an advantage: speed. In order to reduce the
2991: number of errors in primitives and to make programming them easier, we
2992: provide a tool, the primitive generator (@file{prims2x.fs}), that
2993: automatically generates most (and sometimes all) of the C code for a
2994: primitive from the stack effect notation. The source for a primitive
2995: has the following form:
2996:
2997: @format
2998: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
2999: [@code{""}@var{glossary entry}@code{""}]
3000: @var{C code}
3001: [@code{:}
3002: @var{Forth code}]
3003: @end format
3004:
3005: The items in brackets are optional. The category and glossary fields
3006: are there for generating the documentation, the Forth code is there
3007: for manual implementations on machines without GNU C. E.g., the source
3008: for the primitive @code{+} is:
3009: @example
3010: + n1 n2 -- n core plus
3011: n = n1+n2;
3012: @end example
3013:
3014: This looks like a specification, but in fact @code{n = n1+n2} is C
3015: code. Our primitive generation tool extracts a lot of information from
3016: the stack effect notations@footnote{We use a one-stack notation, even
3017: though we have separate data and floating-point stacks; The separate
3018: notation can be generated easily from the unified notation.}: The number
3019: of items popped from and pushed on the stack, their type, and by what
3020: name they are referred to in the C code. It then generates a C code
3021: prelude and postlude for each primitive. The final C code for @code{+}
3022: looks like this:
3023:
3024: @example
3025: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
3026: /* */ /* documentation */
1.4 anton 3027: @{
1.3 anton 3028: DEF_CA /* definition of variable ca (indirect threading) */
3029: Cell n1; /* definitions of variables */
3030: Cell n2;
3031: Cell n;
3032: n1 = (Cell) sp[1]; /* input */
3033: n2 = (Cell) TOS;
3034: sp += 1; /* stack adjustment */
3035: NAME("+") /* debugging output (with -DDEBUG) */
1.4 anton 3036: @{
1.3 anton 3037: n = n1+n2; /* C code taken from the source */
1.4 anton 3038: @}
1.3 anton 3039: NEXT_P1; /* NEXT part 1 */
3040: TOS = (Cell)n; /* output */
3041: NEXT_P2; /* NEXT part 2 */
1.4 anton 3042: @}
1.3 anton 3043: @end example
3044:
3045: This looks long and inefficient, but the GNU C compiler optimizes quite
3046: well and produces optimal code for @code{+} on, e.g., the R3000 and the
3047: HP RISC machines: Defining the @code{n}s does not produce any code, and
3048: using them as intermediate storage also adds no cost.
3049:
3050: There are also other optimizations, that are not illustrated by this
3051: example: Assignments between simple variables are usually for free (copy
3052: propagation). If one of the stack items is not used by the primitive
3053: (e.g. in @code{drop}), the compiler eliminates the load from the stack
3054: (dead code elimination). On the other hand, there are some things that
3055: the compiler does not do, therefore they are performed by
3056: @file{prims2x.fs}: The compiler does not optimize code away that stores
3057: a stack item to the place where it just came from (e.g., @code{over}).
3058:
3059: While programming a primitive is usually easy, there are a few cases
3060: where the programmer has to take the actions of the generator into
3061: account, most notably @code{?dup}, but also words that do not (always)
3062: fall through to NEXT.
3063:
1.4 anton 3064: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1.3 anton 3065: @subsection TOS Optimization
3066:
3067: An important optimization for stack machine emulators, e.g., Forth
3068: engines, is keeping one or more of the top stack items in
1.4 anton 3069: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
3070: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1.3 anton 3071: @itemize
3072: @item
3073: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
3074: due to fewer loads from and stores to the stack.
3075: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
3076: @var{y<n}, due to additional moves between registers.
3077: @end itemize
3078:
3079: In particular, keeping one item in a register is never a disadvantage,
3080: if there are enough registers. Keeping two items in registers is a
3081: disadvantage for frequent words like @code{?branch}, constants,
3082: variables, literals and @code{i}. Therefore our generator only produces
3083: code that keeps zero or one items in registers. The generated C code
3084: covers both cases; the selection between these alternatives is made at
3085: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
3086: code for @code{+} is just a simple variable name in the one-item case,
3087: otherwise it is a macro that expands into @code{sp[0]}. Note that the
3088: GNU C compiler tries to keep simple variables like @code{TOS} in
3089: registers, and it usually succeeds, if there are enough registers.
3090:
3091: The primitive generator performs the TOS optimization for the
3092: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
3093: operations the benefit of this optimization is even larger:
3094: floating-point operations take quite long on most processors, but can be
3095: performed in parallel with other operations as long as their results are
3096: not used. If the FP-TOS is kept in a register, this works. If
3097: it is kept on the stack, i.e., in memory, the store into memory has to
3098: wait for the result of the floating-point operation, lengthening the
3099: execution time of the primitive considerably.
3100:
3101: The TOS optimization makes the automatic generation of primitives a
3102: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
3103: @code{TOS} is not sufficient. There are some special cases to
3104: consider:
3105: @itemize
3106: @item In the case of @code{dup ( w -- w w )} the generator must not
3107: eliminate the store to the original location of the item on the stack,
3108: if the TOS optimization is turned on.
1.4 anton 3109: @item Primitives with stack effects of the form @code{--}
3110: @var{out1}...@var{outy} must store the TOS to the stack at the start.
3111: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1.3 anton 3112: must load the TOS from the stack at the end. But for the null stack
3113: effect @code{--} no stores or loads should be generated.
3114: @end itemize
3115:
1.4 anton 3116: @node Produced code, , TOS Optimization, Primitives
1.3 anton 3117: @subsection Produced code
3118:
3119: To see what assembly code is produced for the primitives on your machine
3120: with your compiler and your flag settings, type @code{make engine.s} and
1.4 anton 3121: look at the resulting file @file{engine.s}.
1.3 anton 3122:
1.17 anton 3123: @node System Architecture, Performance, Primitives, Internals
1.3 anton 3124: @section System Architecture
3125:
3126: Our Forth system consists not only of primitives, but also of
3127: definitions written in Forth. Since the Forth compiler itself belongs
3128: to those definitions, it is not possible to start the system with the
3129: primitives and the Forth source alone. Therefore we provide the Forth
3130: code as an image file in nearly executable form. At the start of the
3131: system a C routine loads the image file into memory, sets up the
3132: memory (stacks etc.) according to information in the image file, and
3133: starts executing Forth code.
3134:
3135: The image file format is a compromise between the goals of making it
3136: easy to generate image files and making them portable. The easiest way
3137: to generate an image file is to just generate a memory dump. However,
3138: this kind of image file cannot be used on a different machine, or on
3139: the next version of the engine on the same machine, it even might not
3140: work with the same engine compiled by a different version of the C
3141: compiler. We would like to have as few versions of the image file as
3142: possible, because we do not want to distribute many versions of the
3143: same image file, and to make it easy for the users to use their image
3144: files on many machines. We currently need to create a different image
3145: file for machines with different cell sizes and different byte order
1.17 anton 3146: (little- or big-endian)@footnote{We are considering adding information to the
1.3 anton 3147: image file that enables the loader to change the byte order.}.
3148:
3149: Forth code that is going to end up in a portable image file has to
1.4 anton 3150: comply to some restrictions: addresses have to be stored in memory with
3151: special words (@code{A!}, @code{A,}, etc.) in order to make the code
3152: relocatable. Cells, floats, etc., have to be stored at the natural
3153: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
3154: dividable by~8. This happens automatically in our system when you use
3155: the ANS Forth alignment words.}, in order to avoid alignment faults on
3156: machines with stricter alignment. The image file is produced by a
3157: metacompiler (@file{cross.fs}).
1.3 anton 3158:
3159: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
3160: file is not directly executable, but has to undergo some manipulations
3161: during loading. Address relocation is performed at image load-time, not
3162: at run-time. The loader also has to replace tokens standing for
3163: primitive calls with the appropriate code-field addresses (or code
3164: addresses in the case of direct threading).
1.4 anton 3165:
1.17 anton 3166: @node Performance, , System Architecture, Internals
3167: @section Performance
3168:
3169: On RISCs the Gforth engine is very close to optimal; i.e., it is usually
3170: impossible to write a significantly faster engine.
3171:
3172: On register-starved machines like the 386 architecture processors
3173: improvements are possible, because @code{gcc} does not utilize the
3174: registers as well as a human, even with explicit register declarations;
3175: e.g., Bernd Beuster wrote a Forth system fragment in assembly language
3176: and hand-tuned it for the 486; this system is 1.19 times faster on the
3177: Sieve benchmark on a 486DX2/66 than Gforth compiled with
3178: @code{gcc-2.6.3} with @code{-DFORCE_REG}.
3179:
3180: However, this potential advantage of assembly language implementations
3181: is not necessarily realized in complete Forth systems: We compared
3182: Gforth (compiled with @code{gcc-2.6.3} and @code{-DFORCE_REG}) with
1.18 ! anton 3183: Win32Forth 1.2093 and LMI's NT Forth (Beta, May 1994), two systems
! 3184: written in assembly, and with two systems written in C: PFE-0.9.11
! 3185: (compiled with @code{gcc-2.6.3} with the default configuration for
! 3186: Linux: @code{-O2 -fomit-frame-pointer -DUSE_REGS}) and ThisForth Beta
! 3187: (compiled with gcc-2.6.3 -O3 -fomit-frame-pointer). We benchmarked
! 3188: Gforth, PFE and ThisForth on a 486DX2/66 under Linux. Kenneth O'Heskin
! 3189: kindly provided the results for Win32Forth and NT Forth on a 486DX2/66
! 3190: with similar memory performance under Windows NT.
1.17 anton 3191:
3192: We used four small benchmarks: the ubiquitous Sieve; bubble-sorting and
3193: matrix multiplication come from the Stanford integer benchmarks and have
3194: been translated into Forth by Martin Fraeman; we used the versions
3195: included in the TILE Forth package; and a recursive Fibonacci number
3196: computation for benchmark calling performance. The following table shows
3197: the time taken for the benchmarks scaled by the time taken by Gforth (in
3198: other words, it shows the speedup factor that Gforth achieved over the
3199: other systems).
3200:
3201: @example
3202: relative Win32- NT This-
3203: time Gforth Forth Forth PFE Forth
3204: sieve 1.00 1.30 1.07 1.67 2.98
3205: bubble 1.00 1.30 1.40 1.66
3206: matmul 1.00 1.40 1.29 2.24
3207: fib 1.00 1.44 1.26 1.82 2.82
3208: @end example
3209:
3210: You may find the good performance of Gforth compared with the systems
3211: written in assembly language quite surprising. One important reason for
3212: the disappointing performance of these systems is probably that they are
3213: not written optimally for the 486 (e.g., they use the @code{lods}
3214: instruction). In addition, Win32Forth uses a comfortable, but costly
3215: method for relocating the Forth image: like @code{cforth}, it computes
3216: the actual addresses at run time, resulting in two address computations
3217: per NEXT (@pxref{System Architecture}).
3218:
3219: The speedup of Gforth over PFE and ThisForth can be easily explained
3220: with the self-imposed restriction to standard C (although the measured
3221: implementation of PFE uses a GNU C extension: global register
3222: variables), which makes efficient threading impossible. Moreover,
3223: current C compilers have a hard time optimizing other aspects of the
3224: ThisForth source.
3225:
3226: Note that the performance of Gforth on 386 architecture processors
3227: varies widely with the version of @code{gcc} used. E.g., @code{gcc-2.5.8}
3228: failed to allocate any of the virtual machine registers into real
3229: machine registers by itself and would not work correctly with explicit
3230: register declarations, giving a 1.3 times slower engine (on a 486DX2/66
3231: running the Sieve) than the one measured above.
3232:
1.4 anton 3233: @node Bugs, Pedigree, Internals, Top
3234: @chapter Bugs
3235:
1.17 anton 3236: Known bugs are described in the file BUGS in the Gforth distribution.
3237:
3238: If you find a bug, please send a bug report to !!. A bug report should
3239: describe the Gforth version used (it is announced at the start of an
3240: interactive Gforth session), the machine and operating system (on Unix
3241: systems you can use @code{uname -a} to produce this information), the
3242: installation options (!! a way to find them out), and a complete list of
3243: changes you (or your installer) have made to the Gforth sources (if
3244: any); it should contain a program (or a sequence of keyboard commands)
3245: that reproduces the bug and a description of what you think constitutes
3246: the buggy behaviour.
3247:
3248: For a thorough guide on reporting bugs read @ref{Bug Reporting, , How
3249: to Report Bugs, gcc.info, GNU C Manual}.
3250:
3251:
1.4 anton 3252: @node Pedigree, Word Index, Bugs, Top
3253: @chapter Pedigree
3254:
1.17 anton 3255: Gforth descends from BigForth (1993) and fig-Forth. Gforth and PFE (by
3256: Dirk Zoller) will cross-fertilize each other. Of course, a significant part of the design of Gforth was prescribed by ANS Forth.
3257:
3258: Bernd Paysan wrote BigForth, a child of VolksForth.
3259:
3260: VolksForth descends from F83. !! Authors? When?
3261:
3262: Laxen and Perry wrote F83 as a model implementation of the
3263: Forth-83 standard. !! Pedigree? When?
3264:
3265: A team led by Bill Ragsdale implemented fig-Forth on many processors in
3266: 1979. Dean Sanderson and Bill Ragsdale developed the original
3267: implementation of fig-Forth based on microForth.
3268:
3269: !! microForth pedigree
3270:
3271: A part of the information in this section comes from @cite{The Evolution
3272: of Forth} by Elizabeth D. Rather, Donald R. Colburn and Charles
3273: H. Moore, presented at the HOPL-II conference and preprinted in SIGPLAN
3274: Notices 28(3), 1993. You can find more historical and genealogical
3275: information about Forth there.
3276:
1.4 anton 3277: @node Word Index, Node Index, Pedigree, Top
3278: @chapter Word Index
3279:
1.18 ! anton 3280: This index is as incomplete as the manual. Each word is listed with
! 3281: stack effect and wordset.
1.17 anton 3282:
3283: @printindex fn
3284:
1.4 anton 3285: @node Node Index, , Word Index, Top
3286: @chapter Node Index
1.17 anton 3287:
3288: This index is even less complete than the manual.
1.1 anton 3289:
3290: @contents
3291: @bye
3292:
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